System and method for inspecting a workpiece surface by analyzing scattered light in a front quartersphere region above the workpiece

ABSTRACT

A surface inspection system, as well as related components and methods, are provided. The surface inspection system includes a beam source subsystem, a beam scanning subsystem, a workpiece movement subsystem, an optical collection and detection subsystem, and a processing subsystem. The optical collection and detection system features, in the front quartersphere, a light channel assembly for collecting light reflected from the surface of the workpiece, and a front collector and wing collectors for collecting light scattered from the surface, to greatly improve the measurement capabilities of the system. The light channel assembly has a switchable edge exclusion mask and a reflected light detection system for improved detection of the reflected light.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims the benefit of the filing date of U.S.Provisional Patent Application No. 60/638,529, filed Dec. 19, 2004,currently pending, the entire disclosure of which is incorporated hereinby reference. The present application is also related to co-pending U.S.patent application Ser. No. 11/311,943 (Ref. No. PAT 2004-010.01), U.S.patent application Ser. No. 11/311,907 (Ref. No. PAT 2004-010.02), U.S.patent application Ser. No. 11/311,908 (Ref. No. PAT2004-010.04), U.S.patent application Ser. No. 11/311,904 (Ref. No. PAT 2004-010.05), U.S.patent application Ser. No. 11/311,905 (Ref. No. PAT 2004-010.06), U.S.patent application Ser. No. 11/311,925 (Ref. No. PAT 2004-010.07), U.S.patent application Ser. No. 11/311,926 (Ref. No. PAT 2004-010.08), andU.S. patent application Ser. No. 11/311,924 (Ref. No. PAT 2004-010.09),all of which were filed on 17 Dec. 2005.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

N/A

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to technology for the inspection of asurface or surfaces of a workpiece, such as a semiconductor wafer, chip,or the like. More particularly, it relates to apparatus and methods forinspection of such workpiece surfaces using electromagnetic energy,e.g., light, to scan the surface to obtain characteristics of thesurface or other information concerning the surface.

2. Description of the Related Art

There are a number of applications in which it is desirable oradvantageous to inspect a surface or surfaces of a workpiece to obtaininformation about the characteristics and/or condition of that surfaceor surfaces. Examples of workpieces amenable to such application wouldinclude, for example, bare or unpatterned semiconductor wafers,semiconductor wafers with an applied film or films, patterned wafers,and the like. Characteristics and conditions of the surface that arecommonly of interest include surface geometry such as flatness, surfaceroughness, etc., and/or the presence of defects, such as particles,crystal originated pits (“COPs”) and crystalline growths. Given theincreasing drive over the years to reduce device size and density, therehas been a need for increasing control over surface characteristics orproperties at reduced dimensions, and an increasing demand for areduction in the size of defects, the types of defects that arepermissible, etc. Correspondingly, there is an enhanced need forresolution, detection and characterization of small surfacecharacteristics, properties, defects, etc., and an enhanced need forincreased measurement sensitivity and classification capability.

In the face of this demand, a number of systems and methods have emergedto provide this capability. One such system, for example, is disclosedin U.S. Pat. No. 5,712,701 (the “'701 patent”), which is assigned to ADEOptical Systems Corporation of Westwood, Massachusetts. The '701 patentdiscloses a surface inspection system and related methods for inspectingthe surface of a workpiece, wherein a beam of laser light is directed tothe surface of the workpiece, the light is reflected off the surface,and both scattered and specular light are collected to obtaininformation about the surface. An acousto-optical deflector is used toscan the beam as the wafer is moved, for example, by combined rotationand translation, so that the entire surface of the workpiece isinspected.

As our understanding of the physics and phenomenology of opticalscattering from surfaces has improved, a capability has been developedand refined in which detailed and high resolution information aboutdefects on the surface can be ascertained. These phenomena largely areobtained from the optical energy that is scattered by the surface, asopposed to the energy in the main reflected beam or the “specular beam.”Examples of systems and methods that provide such defect detectioncapability include that of the '701 patent, as well as U.S. Pat. Nos.6,118,525 and 6,292,259, all of which are assigned to ADE OpticalSystems Corporation and all of which are herein incorporated byreference. Systems designed according to these patents have performedadmirably and provided major advances over their predecessors. As thedrive to smaller device dimensions and higher device densities hascontinued, however, the need also has continued for the ability toresolve and classify even smaller and smaller surface properties,defects, etc. A need also has developed to detect and characterize agreater range of surface characteristics and defects in terms of thetypes of defects, their extent or range, etc. Surface scratches are anexample. Scratches on the surface of a workpiece often do not lie alonga straight line. Surface scratches on semiconductor wafer surfaces, forexample, can be the result of polishing, which can leave circular,curved or irregular scratch geometry. As the workpiece surface is movedrelative to the beam, the orientation of the scratch relative to theoblique incident beam and collectors changes. This often causes changesin the amplitude and direction of scattered light from the scratch asthe wafer rotates. As device dimensions decrease, the ability to detectand characterize the scratches and similar defects with improvedsensitivity and reliability has become increasingly important.

Systems that are amenable to inspection and measurement of extremelysmall dimensions typically must operate in extremely clean environments.This commonly requires that they be contained and operated within aclean room. This highly controlled environment limits normal access tosuch machines and systems, which increases the difficulty and expense oftheir maintenance and repair. Accordingly, systems and methods areneeded that are amenable to more efficient and effective replacement ofprecision-aligned optical sub-components within the machines.

OBJECTS OF THE INVENTION

Accordingly, an object of the present invention according to one aspectis to provide apparatus and methods for inspecting a surface of aworkpiece with high sensitivity and reliability, e.g., for surfacedefects.

Another object of the invention according to another aspect is toprovide apparatus and methods for inspecting a surface of a workpiecethat enable an improved range of detection for surface characteristics,such as defects, defect type, etc., relative to known systems andmethods.

Another object of the invention according to another aspect is toprovide apparatus and methods for inspecting a surface of a workpiecethat are accessible and/or amenable to efficient maintenance, repair,upgrade, and the like.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations pointed out in the appendedclaims.

SUMMARY OF THE INVENTION

To achieve the foregoing objects, and in accordance with the purposes ofthe invention as embodied and broadly described in this document, asurface inspection system is provided for inspecting the surface of aworkpiece. The surface inspection system comprises a base, a beam sourcesubsystem, a beam scanning subsystem, a workpiece movement subsystem, anoptical collection and detection subsystem, and a processing subsystem.The beam source subsystem comprises a beam source that projects anincident beam toward the surface of the workpiece. The beam scanningsubsystem comprises means for receiving the incident beam and scanningthe incident beam on the surface of the workpiece. The workpiecemovement subsystem moves the surface of the workpiece relative to theincident beam. The optical collection and detection subsystem collectsportions of the incident beam that are reflected or scattered from thesurface of the workpiece and generates signals in response to thereflected portions of the incident beam. The processing subsystem isoperatively coupled to the collection and detection subsystem forprocessing the signals.

Optionally but preferably, the beam source module comprises a beamsource housing for fixedly supporting the beam source, and a beam sourcemounting means for fixedly mounting the beam source housing relative tothe base so that the incident beam is projected at a pointing angle to apointing position that is within about 50 micro radians of a target spotcorresponding to a desired spot on the surface of the workpiece. Inaddition, again optionally but preferably, the beam source modulefurther comprises means for pre-aligning the incident beam to thepointing angle and the pointing position. In a presently preferredembodiment, the beam source housing mounting means comprises a pluralityof pinholes and the beam scanning mounting means comprises acorresponding plurality of pins that mate with the plurality ofpinholes. In another, the beam source housing mounting means comprises aplurality of pins and the beam scanning mounting means comprises acorresponding plurality of pinholes that mate with the plurality ofpins. The beam source housing also may comprise a plurality of pinholes,and the beam scanning mounting means may comprise a correspondingplurality of pins that mate with the plurality of pinholes.

The beam scanning module preferably comprises a beam scanning modulehousing for supporting the beam scanning means, and beam scanningmounting means for fixedly mounting the beam scanning module housingrelative to the base so that the beam is projected to a pointingposition that is within about 50 micro radians of a target spotcorresponding to a desired spot on the surface of the workpiece. Thebeam scanning module also preferably comprises means for pre-aligningthe incident beam to the pointing position. In a presently preferredembodiment, the base comprises a plurality of pinholes and the beamscanning mounting means comprises a corresponding plurality of pins thatmate with the plurality of pinholes. In another embodiment, the basecomprises a plurality of pins and the beam scanning mounting meanscomprises a corresponding plurality of pinholes that mate with theplurality of pins.

The optical collection and detection module preferably comprises acollection and detection module housing for supporting the opticalcollection and detection module, and collection and detection modulemounting means for fixedly mounting the collection and detection moduleto the base. In a presently preferred embodiment, the base comprises aplurality of pinholes and the collection and detection module mountingmeans comprises a corresponding plurality of pins that mate with theplurality of pinholes. In another, the base comprises a plurality ofpins and the collection and detection module mounting means comprises acorresponding plurality of pinholes that mate with the plurality ofpins.

The collection and detection module, also known as collector detectormodule, preferably comprises at least one, and preferably two, wingcollectors positioned to collect the portions of the incident beam thatare scattered from the surface of the workpiece. The wing collector orwing collectors are disposed in a front quartersphere, outside anincident plane defined by the incident beam and a light channel axis,and at or near a maximum of the signal to noise ratio. According toanother aspect, the wing collector or wing collectors are positioned innull, or a local minimum, in surface roughness scatter relative todefect scatter, for example, from a defect perspective, at a maximum inthe signal to noise ratio of defect scatter to surface roughness scatterwhen the incident beam is P polarized, or, from a surface roughnessscatter perspective, when the surface roughness is at a relative minimumin a bi-directional reflectance distribution function when the incidentbeam is P polarized.

In accordance with another aspect of the invention, a method is providedfor assembling a surface inspection system having a base. The methodcomprises providing the base to include a first mating device, providinga beam source subsystem having a beam source that projects an incidentbeam and a beam source housing having a second mating device, whereinthe beam source is mounted to the beam source housing, pre-aligning thebeam source relative to the beam source housing prior to placement ofthe beam source housing on the base so that the incident beam isprojected at a pointing angle to a pointing position that is withinabout 50 micro radians of a target spot corresponding to a desired spoton the surface of the workpiece, and positioning the beam source housingon the base using the first and second mating devices, whereby the firstand second mating devices automatically cause the incident beam to be inthe pointing position.

In presently preferred implementations of this method, the first matingdevice may comprise a plurality of pinholes, and the second matingdevice comprises a plurality of pins that mate with the plurality ofpinholes. The method also may be implemented so that the first matingdevice comprises a plurality of pins, and the second mating devicecomprises a plurality of pinholes that mate with the plurality of pins.

In accordance with another aspect of the invention, a method is providedfor assembling a surface inspection system having a base. The methodcomprises providing a base having a first mating device, and providing abeam source subsystem having a beam source that projects an incidentbeam and a beam source housing having a second mating device, whereinthe beam source is mounted to the beam source housing. The method alsocomprises providing a beam scanning subsystem having a beam scanningdevice that scans an incident beam on the surface of the workpiece,wherein the beam scanning subsystem comprises a beam scanning subsystemhousing having third and fourth mating devices, and wherein the beamscanning device is mounted to the beam scanning housing. The methodfurther comprises pre-aligning the beam source relative to the beamsource housing prior to placement of the beam source housing on the beamscanning subsystem housing so that the incident beam is projected at apointing angle to a pointing position that is within about 50 microradians of a target spot corresponding to a desired spot on the surfaceof the workpiece. The method still further comprises pre-aligning thebeam scanning device relative to the beam scanning subsystem housingprior to placement of the beam scanning subsystem housing on the base sothat the incident beam is projected to the pointing position. Thismethod also comprises positioning the beam source housing on the beamscanning subsystem housing using the second and third mating devices,whereby the second and third mating devices automatically cause theincident beam to be in the pointing position and at the pointing angle.It also comprises positioning the beam scanning housing on the baseusing the first and fourth mating devices, whereby the first and fourthmating devices automatically cause the incident beam to be in thepointing position.

In accordance with another method according to the invention, a basehaving a first mating device is provided, as is a beam scanningsubsystem having a beam scanning device that scans an incident beam onthe surface of the workpiece. The beam scanning subsystem comprises abeam scanning subsystem housing to which the beam scanning device ismounted. The beam scanning housing comprises a second mating device. Themethod also comprises pre-aligning the beam scanning device relative tothe beam scanning subsystem housing prior to placement of the beamscanning subsystem housing on the base so that the incident beam isproject to a pointing position that is within about 50 micro radians ofa target spot corresponding to a desired spot on the surface of theworkpiece. The method further comprises positioning the beam scanningsubsystem housing on the base using the first and second mating devices,whereby the first and second mating devices automatically cause theincident beam to be in the pointing position.

In implementing this method, one may provide the first mating device tocomprise a plurality of pinholes, and the second mating device may beprovided to comprise a plurality of pins that mate with the plurality ofpinholes. In another implementation, the first mating device comprises aplurality of pins, and the second mating device comprises a plurality ofpinholes that mate with the plurality of pins.

In accordance with another aspect of the invention, a method is providedfor assembling a surface inspection system having a base. The methodcomprises a base having a first mating device, and providing acollection and detection subsystem that comprises a collector module anda detector module mounted to a collection and detection subsystemhousing for supporting the optical and detection subsystem. Prior toplacement of the collection and detection subsystem housing on the base,the method includes pre-aligning the collector module and the detectormodule to receive reflected portions of an incident beam reflected fromthe surface of the workpiece. The method also includes positioning thecollection and detection subsystem housing on the base using the firstand second mating devices, whereby the first and second mating devicesautomatically cause the collector module and the detector module to bepositioned to receive the reflected portions of the incident beam. In apresently preferred implementation, the first mating device comprises aplurality of pinholes, and the second mating device comprises aplurality of pins that mate with the plurality of pinholes. In anotherimplementation, the first mating device comprises a plurality of pins,and the second mating device comprises a plurality of pinholes that matewith the plurality of pins.

In accordance with another aspect of the invention, a method is providedfor affixing a beam scanning subsystem to a surface inspection systemfor inspecting a surface of a workpiece. The method comprises providinga beam scanning module, which beam scanning module scans a beam. Afterproviding the beam scanning module, the method includes pre-aligning thebeam as it is projected from the beam scanning module so that the beamis projected to a pointing position that is within about 50 microradians of a target spot corresponding to a desired spot on theworkpiece. After this pre-alignment, the method includes fixedlymounting the beam scanning module with the pre-aligned beam to relativethe base so that the beam automatically remains pre-aligned to thepointing position. The beam scanning module preferably is mounted to thebase and is detachable. The mounting may be accomplished using matingpins and pinholes to mount the beam scanning module.

In accordance with still another aspect of the invention, a method isprovided for affixing a collection and detection module to a surfaceinspection system for inspecting a surface of a workpiece. The methodcomprises providing the collection and detection module that comprises acollector and a detector module which respectively collect and detectlight of a beam reflected from the surface. After providing thecollection and detection module, the method includes pre-aligning thecollection and detection module so that the collector module anddetector module are at desired positions along a collection axis desiredspot on the workpiece. After this pre-alignment, the method includesmounting the pre-aligned collection and detection module relative to thebase so that the collector and the detector module remain pre-aligned tothe desired positions. In preferred implementations of this method, themounting is detachable. The mounting may comprise the use of mating pinsand pinholes to mount the collection and detection module.

In accordance with another aspect of the invention, a method is providedfor maintaining a surface inspection system used for inspecting asurface of a workpiece. The surface inspection system has a first beamsource module coupled to a base. The method comprises de-coupling andremoving the first beam source module from the base, and providing asecond beam source module, which second beam source module projects abeam. After these, the method comprises pre-aligning the beam as it isprojected from the second beam source module so that the beam isprojected to a pointing position that is within about 50 micro radiansof a target spot corresponding to a desired spot on the workpiece, andafter performing these, mounting the housing with the pre-aligned beamto the base so that the beam automatically remains pre-aligned to thepointing position. The mounting may comprise using mating pins andpinholes to mount the housing to the base so that that beam is in thepointing position.

In accordance with another aspect of the invention, a method is providedfor maintaining a surface inspection system used for inspecting asurface of a workpiece, wherein the surface inspection system has afirst beam source module coupled to a beam scanning housing. The methodcomprises de-coupling and removing the first beam source module from thebeam scanning housing, and providing a second beam source module, whichsecond beam source module projects a beam. After performing these, themethod includes pre-aligning the beam as it is projected from the secondbeam source module so that the beam is projected to a pointing positionthat is within about 50 micro radians of a target spot corresponding toa desired spot on the workpiece. This is performed by adjusting theposition of the beam scanning module with respect to the target spot.After performing these, the method includes mounting the second beamsource module with the pre-aligned beam to the beam scanning housing sothat the beam automatically remains pre-aligned to the pointingposition. The mounting may comprise using mating pins and pinholes tomount the housing to the beam scanning housing so that that beam is inthe pointing position.

In accordance with yet another aspect of the invention, a method isprovided for maintaining a surface inspection system used for inspectinga surface of a workpiece. The surface inspection system has a first beamscanning module coupled to a base. The method comprises de-coupling andremoving the first beam scanning module from the base, and providing asecond beam scanning module, which second scanning beam source scans abeam. After performing these, the method includes pre-aligning the beamas it is projected from the beam scanning module so that the beam isprojected to a pointing position that is within about 50 micro radiansof a target spot corresponding to a desired spot on the workpiece. Afterperforming this, the method includes mounting the beam scanning modulewith the pre-aligned beam to the base so that the beam automaticallyremains pre-aligned to the pointing position. The mounting may compriseusing mating pins and pinholes to mount the housing to the base so thatthat beam is in the pointing position.

In accordance with another aspect of the invention, a variable scanningspeed acousto-optical deflector assembly is provided. It comprises anacousto-optical deflector, means operatively coupled to theacousto-optical deflector for varying the scan speed at which theacousto-optical deflector scans a beam passing through theacousto-optical deflector, and beam compensating means for compensatingfor astigmatism of the beam associated with the variation of scan speed.

In preferred embodiments of the variable scanning speed acousto-opticaldeflector assembly, a scanning speed selection device operativelycoupled to the acousto-optical deflector selects one of a plurality ofscan speeds, and compensating optics compensate for astigmatism of thebeam associated with the variation of scan speed. The compensatingoptics comprise a plurality of lenses and a lens positioning deviceoperatively coupled to the plurality of lenses, for example, asdescribed above, wherein the lens positioning device positions aselected one of the lenses in the beam at the output of theacousto-optical deflector, and each of the lenses provides a uniqueamount of compensation relative to others of the lenses. As noted,cylindrical lenses are optional but preferred.

The beam compensating means preferably comprises a plurality of lensesand a lens positioning device operatively coupled to the plurality oflenses, and the lens positioning device positions a selected one of thelenses in the beam at an output of the acousto-optical deflector. Inthis event, each of the lenses preferably causes the acousto-opticaldeflector to provide a unique amount of compensation relative to othersof the lenses. The lenses in the plurality of lenses preferably comprisecylindrical lenses. Preferably there are two lenses, although this isnot necessarily limiting and more such lenses may be provided. It alsois preferred that the focal lengths of the lenses differ. The lenspositioning device may comprise a housing for the plurality of lenses,wherein the housing moves the respective lenses into and out of thebeam. The variable speed scanning device may include a pneumaticpressure source for moving the respective lenses into and out of thebeam. The lenses may be rotated in and out using a carousel arrangement,or may be exchanged using a slide mechanism.

In a presently preferred embodiment, each of the lenses comprises acylindrical lens having a longitudinal lens axis, the lens positioningdevice housing holds the lenses so that the longitudinal lens axes aresubstantially aligned, and the lens positioning device moves thecylindrical lenses in a direction parallel to the longitudinal lensaxes. In another presently preferred embodiment, each of the lensescomprises a cylindrical lens having a longitudinal lens axis, and thelens positioning device housing moves the cylindrical lenses by rotatingthe respective lenses into the beam. In each of these preferredembodiments, it also is preferred that the lenses are positionedimmediately adjacent to the acousto-optical deflector.

In accordance with another aspect of the invention, a method is providedfor scanning a surface of a workpiece. The method comprises using anacousto-optic deflector to scan a beam on the surface of the workpieceat a first scanning speed, selecting a second scanning speed differentthan the first scanning speed, using the acousto-optic deflector to scanthe beam on the surface of the workpiece at the second scanning speed,and compensating for changes to the beam caused by scanning at thesecond scanning speed relative to the first scanning speed. Thistypically will involve compensating for astigmatism of the beamassociated with the change from the first scanning speed to the secondscanning speed. The compensating preferably comprises selectivelypositioning a selected one of a plurality of lenses in the beam at theoutput of the acousto-optical deflector, wherein each of the lensesprovides a unique amount of compensation relative to others of thelenses. This also preferably comprises using cylindrical lenses, andpreferably at least two such lenses, each having a focal length that isunique relative to others lenses of the plurality of lenses. Thecompensating preferably comprises moving the respective lenses and thebeam relative to one another so that one of the respective lenses ispositioned within the beam. This preferably comprises moving therespective lenses, e.g., longitudinally or rotating the lenses into andout of the beam.

In accordance with another aspect of the invention, an opticalcollection system is provided for use in a surface inspection system forinspecting a surface of a workpiece. The surface inspection system hasan incident beam projected through a back quartersphere and toward adesired spot on the surface of the workpiece so that a specular portionof the incident beam is reflected along a light channel axis in a frontquartersphere. Inspection systems that comprise the optical collectionsystem according to this aspect of the invention comprise an additionalaspect of the invention. The incident beam and the light channel axisform an incident plane. The optical collection system according to thisaspect of the invention comprises at least one wing collector positionedto collect a scattered portion of the incident beam. The wing collectoror wing collectors are disposed in the front quartersphere, outside theincident plane, and at a maximum of the signal to noise ratio when theincident beam is P polarized and the collector incorporates aP-polarizing polarizer. The wing collectors also may be positioned inthe front quartersphere, outside the incident plane, and at a null or alocal minimum, in surface roughness scatter relative to defect scatter,for example, from a defect perspective, at a maximum in the signal tonoise ratio of defect scatter to surface roughness scatter when theincident beam is P polarized, or, from a surface roughness scatterperspective, when the surface roughness is at a relative minimum in abi-directional reflectance distribution function.

In presently preferred embodiments according to this aspect of theinvention, the signal comprises a P-polarization component, and the wingcollector is disposed at least one of the maximum of the signal to noiseratio of the P-polarization component, the null of the P-polarizationcomponent of the bi-directional distribution function, and/or theminimum, or a local minimum, of the P-polarization component of thatfunction, when the incident beam is P polarized. This may and preferablyis accomplished using a polarization analyzer orthogonal to thepolarization of the surface roughness scatter.

It is preferred that two such wing collectors be used, although this isnot necessarily limiting. Where two or more wing collectors are used, itis preferred but not required that they be substantially identical. Italso is optional but preferred that they be located symmetrically withrespect to the incident plane, and/or equidistant from the desired spotand/or from the surface. In presently preferred embodiments, a firstwing collector has an azimuth angle with respect to the light channelaxis of about 5 to 90 degrees, and a second wing collector has anazimuth angle with respect to the light channel axis of about −5 to −90degrees. In these embodiments, the first wing collector has an elevationangle with respect to the surface of the workpiece of about 30 to 90degrees, and the second wing collector also has an elevation angle ofabout 30 to −90 degrees. It is more preferred that the first wingcollector has an elevation angle with respect to the surface of theworkpiece of about 45 degrees and the second wing collector also has anelevation angle of about 45 degrees. In the presently preferredembodiments according to this aspect of the invention, each of the firstand second wing collectors has a collection angle of up to about 40°,and more preferably the collection angle is about 26°. In the presentlypreferred embodiments, the optical collection further comprises apolarizing beamsplitter disposed in an optical path of the incident beambetween the desired spot and at least one of the wing collectors. Inthese embodiments, the system further comprises a light channelcollector positioned in the incident plane to receive the specularportion of the incident beam, and a central collector. These collectorsmay be positioned and configured, for example, as is described in the'701 patent. The system also preferably comprises at least one backcollector, as will be described more fully herein below.

In accordance with another aspect of the invention, a method is providedfor inspecting a surface of a workpiece. The method comprises scanningan incident beam on the surface of the workpiece so that a specularportion of the incident beam is reflected along a light channel axis ina front quartersphere, the incident beam and the light channel axisdefining an incident plane. The method also comprises collecting aportion of the scattered light beam at a wing collector disposed in thefront quartersphere, outside the incident plane, and at least one of amaximum of the signal to noise ratio, and/or null or a minimum insurface roughness scatter relative to defect scatter, for example, froma defect perspective, at a maximum in the signal to noise ratio ofdefect scatter to surface roughness scatter when the incident beam is Ppolarized, or, from a surface roughness scatter perspective, when thesurface roughness is at a relative minimum in a bidirectionalreflectance distribution function when the incident beam is P polarized.The method further comprises detecting the collected portions of theincident beam that are reflected from surface of the workpiece andgenerating signals in response, and processing the signals to obtaininformation about the surface. The beam scanning preferably comprisesdirecting the incident beam through a back quartersphere and toward thedesired spot on the surface of the workpiece at an oblique angle withrespect to the surface.

In accordance with still another aspect of the invention, an opticalcollection system is provided for use in a surface inspection system forinspecting a surface of a workpiece. The surface inspection system hasan incident beam projected through a back quartersphere and toward aspot on the surface of the workpiece so that a specular portion of theincident beam is reflected along a light channel axis in a frontquartersphere. As noted herein above, the incident beam and the lightchannel axis form an incident plane. The optical collection systemaccording to this aspect of the invention comprises a plurality of backcollectors positioned in the back quartersphere for collecting scatteredportions of the incident beam. In presently preferred embodimentsaccording to this aspect of the invention, the plurality of backcollectors consists of two back collectors. Preferably the collectors inthe plurality of back collectors are positioned outside the incidentplane. It also is optional but preferred that the plurality of backcollectors are substantially identical. It also is optional butpreferred that the two back collectors are located symmetrically withrespect to the incident plane, and preferably equidistant from theincident plane and/or from the surface of the workpiece. In presentlypreferred embodiments, the two back collectors are positioned at anazimuth angle of up to about 90° with respect to the incident beam, morepreferably at an azimuth angle of about 10 to about 90° with respect tothe incident beam, and even more preferably at an azimuth angle of atleast about 45° to 55° with respect to the incident beam. In thepreferred embodiment described more fully herein below, two backcollectors are positioned at an azimuth angle of about 55° with respectto the incident beam. The back collectors preferably have an elevationangle with respect to the desired spot on the surface of the workpieceof about 55°. In presently preferred embodiments, each of the backcollectors has a collection angle of about 20 to about 60, and morepreferably they have a collection angle of about 30.

In presently preferred embodiments according to this aspect of theinvention, the system optionally comprises a polarizing beam splitterdisposed in an optical path of the beam between the desired spot andeach of the back collectors. Such embodiments also optionally butpreferably comprise a light channel collector positioned in the incidentplane to receive the specular portion of the incident beam, and acentral collector.

Optical collection systems according to this aspect of the invention maybe provided individually, or as part of a surface inspection system.

In accordance with yet another aspect of the invention, a method isprovided for inspecting a surface of a workpiece. The method comprisesscanning an incident beam on the surface of the workpiece so that aspecular portion of the incident beam is reflected along a light channelaxis in a front quartersphere, wherein the incident beam and the lightchannel axis define an incident plane. The method further comprisescollecting scattered portions of the incident beam at a plurality ofback collectors disposed in the back quartersphere, detecting thecollected portions of the scatter and generating signals in response,and processing the signals to obtain information about the surface.

In presently preferred implementations of this method, one or more backcollectors as described above, and as more fully described herein below,are used to collect scattered light from the surface of the workpiece.It is optional but preferred that the beam scanning comprises directingthe incident beam through a back quartersphere and toward the desiredspot on the surface of the workpiece at an oblique angle with respect tothe vector normal to the surface.

In accordance with still another aspect of the invention, a surfaceinspection system is provided for inspecting a surface of a workpiece.The surface inspection system according to this aspect of the inventioncomprises an illumination subsystem that projects a beam to the surfaceof the workpiece. The beam comprises a collimated portion and anon-collimated portion. The illumination subsystem comprises an absorberfor absorbing the non-collimated portion of the beam. The system alsocomprises a collection subsystem for collecting scattered portions ofthe beam scattered by the surface of the workpiece, and a processingsubsystem operatively coupled to the collection subsystem for processingsignals received from the collection subsystem to provide informationabout the surface of the workpiece.

The illumination subsystem preferably comprises an acousto-opticdeflector or the like having an output, and the absorber preferably ispositioned at the output of the acousto-optic deflector or itsequivalent or substitute. The absorber preferably comprises baffling.

In accordance with another aspect of the invention, a surface inspectionsystem is provided for inspecting a surface of a workpiece. The surfaceinspection system comprises an illumination subsystem that projects anincident beam to the surface of the workpiece, wherein the incident beamafter interacting with the surface comprises a reflected (light channel)portion and a scattered (dark channel) portion. The system comprises alight channel that receives the reflected portion of the incident beam.The light channel comprises a beam receiving input and an attenuator atthe beam receiving input. The system further comprises a collectionsubsystem that collects the scattered portion of the incident beam andgenerates signals in response, and a processing subsystem operativelycoupled to the collection subsystem that processes the signals receivedfrom the collection subsystem to provide information about the surfaceof the workpiece. In related aspects of the invention, noise attenuatingfeatures may be provided, for example, by including baffling or the likeat or in the optical path about objective lenses in the system, addingglare stops, and the like.

In accordance with another aspect of the invention, a surface inspectionsystem is provided for inspecting a surface of a workpiece. The surfaceinspection system comprises an illumination subsystem that projects abeam to the surface of the workpiece. The illumination subsystemcomprises a plurality of lenses, wherein each of the lenses has asurface roughness of that does not exceed a desired maximum roughness(such as about 5 Angstroms). The system further comprises a collectionsubsystem for collecting scattered portions of the beam scattered fromthe surface of the workpiece, and a processing subsystem operativelycoupled to the collection subsystem for processing signals received fromthe collection subsystem to provide information about the surface of theworkpiece. In addition or alternatively, the collection subsystemcomprises a plurality of collection lenses, each of the collectionlenses having a surface roughness of that does not exceed a desiredmaximum roughness (such as about 5 Angstroms).

Other aspects of the invention also are included herein and are furtherdescribed herein below. These include, for example, a polarizingbeamsplitter/analyzer, a virtual mask, and a switchable edge exclusionmask. In addition, processing subsystems and related methods areprovided for processing signals obtained from a surface inspectionsystem, and for obtaining useful information from the collected light.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and constitute apart of the specification, present illustrative but not presentlypreferred embodiments and methods of the invention and, together withthe general description given above and the detailed description of theembodiments and methods given below, serve to explain the principles ofthe invention.

Other advantages will appear as the description proceeds when taken inconnection with the accompanying drawings, in which:

FIG. 1 is a perspective view of components of a surface inspectionsystem according to a presently preferred embodiment of one aspect ofthe invention;

FIG. 2 is a top or plan view of the components shown in FIG. 1;

FIG. 3 is a front perspective view of the system, the components ofwhich are shown in FIG. 1, with the system contained in its cabinet;

FIG. 4 shows a back view of the system shown in FIG. 3;

FIGS. 5-7 are diagrams illustrating reference geometry to aid in thedescription of the system shown in FIG. 1;

FIG. 8 is a perspective view of the base plate for the system of FIG. 1;with system components;

FIG. 9 is a side view of the base plate of FIG. 8;

FIG. 10 provides a perspective view of the base plate of FIGS. 8 and 9,without system components;

FIG. 11 is an exploded perspective view of the base plate, beam sourcemodule, beam scanning module and light channel assembly for the systemof FIG. 1;

FIG. 12 is a perspective view of the base plate for the system of FIG. 1with the beam source module and the beam scanning module mounted to thebase;

FIG. 13 is a pictoral diagram of the optics of the beam source module;

FIG. 14 is a top view of the beam scanning module of FIG. 11;

FIG. 15 is a perspective view of the AOD assembly of the beam scanningmodule of the system of FIG. 1;

FIG. 16 is a cutaway view of a portion of the AOD assembly for the beamscanning module;

FIG. 17 is a top or plan view of the AOD assembly for the beam scanningmodule;

FIG. 18 is a diagram of an alternative embodiment of the beamcompensation means for the variable speed beam scanning device accordingto the present invention;

FIG. 19 is a perspective view of the collection and detection subsystemmodule for the system of FIG. 1;

FIG. 20 is a side cutaway view of the module shown in FIG. 19;

FIG. 21 is a perspective view of the base for the system of FIG. 1, withthe collection and detection subsystem module attached;

FIG. 22 is a bottom view of the collection optics of the collection anddetection subsystem for the system of FIG. 1;

FIG. 23-24 are perspective views of the collector-detector assembly;

FIG. 25 is a side cutaway view of a collector and detector assembly;

FIG. 26 is a perspective view of a partially assembled collector anddetector assembly of FIG. 23-25;

FIG. 27 is a front cutaway view of the collector and detector assembly,showing the operation of polarizing beam-splitters according to presentinvention;

FIG. 28 is an alternative preferred embodiment of the polarizingbeamsplitter according to the invention;

FIG. 29 is a top cutaway view of the light channel optics;

FIG. 30 is a graphical illustration of the bi-directional reflectancedistribution function; using linear intensity, for P-polarized lightincident on the workpiece surface from an angle of 65 relative to thenormal, and for a P-polarized receiver;

FIG. 31 is a graphical illustration of the bi-directional reflectancedistribution function, using linear intensity, for P-polarized lightincident on the workpiece surface from an angle of 65 relative to thenormal, and for a P-polarized receiver;

FIG. 32 is a scratch scatter distribution diagram;

FIG. 33 is a bottom view of the collection and detection module for thesystem of FIG. 1, and shows the switchable edge exclusion mask accordingto another aspect of the invention;

FIG. 34 is a side view ray trace through the collection optics whichshows the effects of a virtual mask in accordance with another aspect ofthe invention;

FIG. 35 is an axial view of the ray trace of FIG. 34;

FIG. 36 is a ray trace through the collection optics of a given one ofthe collectors;

FIGS. 37-38 are scratch scatter distribution diagrams;

FIG. 39 is a block diagram of the collectors used in a surfaceinspection system according to the present invention;

FIG. 40 is a block diagram showing one embodiment of a method fordetecting the presence of defects;

FIG. 41 is a block diagram showing another embodiment of the method fordetecting the presence of defects;

FIG. 42 a is a block diagram illustrating components involved in thecombining step of FIG. 41;

FIG. 42 b is a block diagram showing the combining step of FIG. 41;

FIG. 43 is a block diagram showing the combining step of FIG. 41 shownin more detail;

FIG. 44 is a block diagram showing a method for defining multiplechannels from a collector or set of collectors;

FIG. 45 is a block diagram showing the multiple channels that may beformed from the output associated with a set of collectors;

FIG. 46 is a block diagram of a data processing system for the currentinvention;

FIG. 47 is a block diagram of channel formation using the dataprocessing system of FIG. 46;

FIG. 48 is a block diagram showing data flow of the present invention;

FIG. 49 is a block diagram showing the systems communication for thepresent invention;

FIG. 50 is a block diagram showing data flow in the Data AcquisitionNodes;

FIG. 51 is a block diagram showing the data flow in the Dark ChannelData Reduction Nodes;

FIG. 52 is an in-scan surface structure spatial frequency plot for usein analyzing the in-scan frequency surface structure spatial response ofcollectors in a surface inspection system of the present invention;

FIG. 53 is a diagram showing the mapping of the ideal response rangesshown in FIG. 52 into a histogram;

FIG. 54 is a chart for use in analyzing the in-scan surface structurespatial frequency response of collectors in a surface inspection systemof the present invention;

FIG. 55 is a histogram derived from the histogram shown in FIG. 53;

FIG. 56 is a diagram showing a beam trace of light through a lensarrangement and the placement of Lyot stop relative to the lensarrangement, according to the present invention;

FIG. 57 is a 2-dimensional voltage map that contains both backgroundnoise and defect signals;

FIG. 58 is a voltage plot of signal for a selected row in FIG. 57;

FIG. 59 is a defect map for FIG. 57, generated by inserting a constantvalue at each pixel position for which the threshold exceeded thevoltage value corresponding to a 50 nm polystyrene latex sphere (PSL)signal peak;

FIGS. 60 and 61 are plots showing the measured distribution (blackpoints) and underlying Gaussian background noise distribution (X marks)of the signals in FIG. 57;

FIG. 62 is a plot showing an expanded scale view of FIG. 61;

FIG. 63 is a graph showing confidence level factors as a function ofvoltage;

FIG. 64 is Confidence Level Factor (CLF) curve generated by applying apolynomial interpolation fit to FIG. 63, as well as setting the lowerend to 0 and the upper end to 1, in which the horizontal axis isvoltage, and the vertical axis is the CLF;

FIG. 65 is a Confidence Level Map created by mapping the voltage valuesin FIG. 57 using the CLF function plotted in FIG. 64;

FIG. 66 is a slice plot corresponding to a selected row in FIG. 65, inwhich the confidence levels of various defects vary across the scan row;

FIG. 67 is another 2-dimensional voltage map that contains bothbackground noise and defect signals;

FIGS. 68 a-68 c are diagrams of examples of scanning geometries usingsub-images that are processed using Confidence Level DetectionProcessing, with FIG. 68 a showing Confidence Level Detection Processingperformed on a series of sub-images that are sequentially positioned inthe X and Y directions, FIG. 68 b showing a cylinder scan geometry, andFIG. 68 c showing the Archimedes spiral scan;

FIG. 69 is a block diagram showing a portion of a system and method fordetection of a light point defect (LPD) greater than a selected size,performed at set up of a surface inspection system;

FIG. 70 is a block diagram showing another portion of a system andmethod for detection of a light point defect (LPD) greater than aselected size, performed at during operation of the surface inspectionsystem;

FIG. 71 is a diagram of an alternative embodiment of the variablepolarization assembly according to the present invention;

FIG. 72 is a block diagram showing some of the relay lens assemblies 490contemplated by the present invention;

FIGS. 73 and 74 are block diagrams showing implementations of thepre-alignment method contemplated by the present invention;

FIG. 75 is a block diagram of a method for determining an extent of acontribution of surface roughness frequencies on the scattering surface;

FIG. 76 is a diagram illustrating the solid angle for collection ofscatter signal at a collector;

FIG. 77 is a block diagram showing methods of analyzing surfacestructure scatter according to the present invention;

FIG. 78 a, FIG. 78 b, and FIG. 78 c are examples of haze maps fordisplaying haze associated with, respectively high, medium, and lowsurface structure spatial frequency ranges of the present invention;

FIG. 79 is an example of a composite haze map of the present invention;

FIG. 80 is a spectral density plot of haze using methods of the presentinvention;

FIG. 81 is a block diagram of the illumination absorbing system 21 ofthe present invention;

FIG. 82 is a block diagram of the scan repetition method 23 of thepresent invention;

FIG. 83 is a block diagram of the scan repetition system 38 of thepresent invention;

FIG. 84 is a block diagram of the AOD assembly shown in FIGS. 15-17;

FIG. 85 is a block diagram of the collector/detector absorbing means 270of the present invention;

FIGS. 86 and 87 are block diagrams of the Confidence Level DetectionProcessing method 502 of the present invention;

FIG. 88 is a block diagram of the Confidence Level Detection Processingmethod step 862 for developing a confidence level factor function;

FIG. 89 is a block diagram of the Confidence Level Detection Processingmethod step 870 for creating a confidence level map;

FIG. 90 is a block diagram of the Confidence Level Detection Processingmethod step 874 for identifying potential defects and their statisticalsignificance;

FIGS. 91 a and 91 b are illustrations of a workpiece surface structure;

FIGS. 92 a and 92 b are illustrations of a the surface height profile ofa model workpiece surface structure;

FIG. 93 a-93 d are diagrams that show regions representative of theamount of and direction of photons scattered from a surface structure,by waveform component;

FIG. 94 is a diagram of the collection space above a workpiece surface,showing collection areas for the collectors 300 in the surfaceinspection system 10 and representative surface structure spatialfrequency ranges associated therewith;

FIG. 95 is a plot showing the distribution of scatter intensity valueson a wafer, by the percentage of a wafer's area at which a scatterintensity value was measured, for the output associated with threecollectors;

FIGS. 96 and 97 are diagrams shown an example of subdividing selectedsurface structure spatial frequency ranges in haze maps;

FIG. 98 is a block diagram showing a multiple spatial frequency hazeanalysis method of the present invention;

FIG. 99 is a block diagram showing one embodiment of a method foranalyzing scatter by its associated surface structure spatial frequencyrange;

FIG. 100 is a block diagram showing one method of forming nominalsurface structure spatial frequency ranges; and

FIG. 101 is a block diagram showing one embodiment of a multiple spatialfrequency haze analysis method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS AND METHODS

Reference will now be made in detail to the presently preferredembodiments and methods of the invention as illustrated in theaccompanying drawings, in which like reference characters designate likeor corresponding parts throughout the drawings. It should be noted,however, that the invention in its broader aspects is not limited to thespecific details, representative devices and methods, and illustrativeexamples shown and described in this section in connection with thepreferred embodiments and methods. The invention according to itsvarious aspects is particularly pointed out and distinctly claimed inthe attached claims read in view of this specification, and appropriateequivalents.

Surface Inspection System

A surface inspection system 10 and related components, modules andsubassemblies in accordance with various aspects of the invention willnow be described. Surface inspection system 10 is designed to inspect asurface S or surfaces of a workpiece W, such as a silicon wafer. Morespecifically, these illustrative embodiments are adapted for inspectionof unpatterned silicon wafers, with or without surface films. Systemsaccording to the invention also would be suitable for inspecting othertypes of surfaces as well. They are particularly well suited forinspecting optically smooth surfaces that at least partially absorb andscatter the incident beam energy. Examples would include glass andpolished metallic surfaces. Wafer W may comprise known wafer designs,such as known 200 millimeter (mm) wafers, 300 mm wafers, and the like.

System 10 is shown from various perspectives in FIGS. 1-4. FIG. 1 showsa side perspective view block diagram of principal components of thesystem. FIG. 2 shows the same type of block diagram, but from a top orplan view. FIG. 3 provides a front perspective view of system 10contained in its cabinet. FIG. 4 shows a back view of system 10 in itscabinet.

System 10 is contained within a cabinet 12. It includes an operatorinterface 14 comprising a keyboard or similar input device 16, a mouseor similar pointing device 18, and a display device 20 such as a videomonitor. Other peripherals may be provided, such as a printer, networkconnection, and the like. An air filtration device 30, such as a HEPAair filter, is provided for removing dust particles and purifying theair to desired specificity. An external wafer handling system 32, alsoknown as robotic wafer handling subsystem 32 and external workpiecehandling system 32, provides workpieces.

With reference to FIG. 4, within cabinet 12 system 10 includes avibration isolation module 40 within a housing 42. It is within thishousing area that the workpieces W, shown in FIGS. 1 and 2, areiteratively inspected, as described more fully herein below.

System 10 includes a workpiece movement subsystem for movement of thewafer relative to the incident scanned beam. The manner of moving theworkpiece may vary, depending upon the application, the overall systemdesign, and other factors. A number of scan patterns, for example, maybe implemented, as is described more fully below. Indeed, in someapplications it may be desirable to move the beam or scanning subsysteminstead of the wafer, i.e., while maintaining the wafer in a stationarylocation. As implemented in system 10, an internal workpiece handlingsubsystem 44, also known as robotic wafer handling subsystem 44 and amotorized γ-θ stage 44, is provided which comprises a scanner gauge, notshown, and robot, not shown, are housed in cabinet 12. This subsystem isconfigured to work in cooperation with external workpiece handlingsystem 32 to receive the workpieces to be inspected. Internal workpiecehandling subsystem 44 comprises a motorized linear stage 46 and a rotarystage 48. It therefore is capable or both rotating and translating theworkpiece (γ-θ), for example, to provide a number of scan patterns. Thispermits the wafer to be scanning in a variety of generally curved pathsthat provide full and efficient coverage of the entire wafer surface. Itenables such scan patterns as concentric cylinder scans, spiral scansand the like. In the preferred embodiments and methods, a “hybrid scan”pattern is used in which the beam travels in a generally helical orArchimedes spiral scan, but in which the beam is oscillated in a seriesof short scans as the spiral is traced out. This pattern is disclosed inU.S. Pat. Nos. 5,712,701, 6,118,525, and 6,292,259, each of which isassigned to ADE Optical Systems Corporation. Subsystem 44 receives theworkpiece and is used to perform appropriate calibration, as well asmoving the workpiece according to one or more desired scan paths.

System 10 also includes appropriate support subsystems, such as a powersupply 50. A processor 52 and data acquisition subsystem 54 also arecontained within cabinet 12, as will be described more fully hereinbelow.

With reference to FIGS. 1 and 2, the workpiece W, which in thisillustrative example is a semiconductor wafer, resides in an inspectionzone IZ within housing 42 during inspection, as will be described morefully herein below. Motorized γ-θ stage 44 is disposed so that theworkpiece under inspection is positioned within this inspection zone IZ.The workpiece W is placed on this stage for inspection and remains thereduring the inspection.

Spatial Reference Frame Information and Nomenclature

To better illustrate the principles of the invention as manifested inthe presently preferred embodiments and methods, some spatial referenceframe information and nomenclature is useful. These geometricrelationships are illustrated in FIGS. 5-7 with reference to FIG. 1. Theplane defined by the inspection stage, and which generally will besubstantially coplanar with the surface of the workpiece, is referred toherein as the “inspection stage plane” or the “base plane” B. The“incident beam vector” IB is the vector or ray along which the incidentbeam propagates between the beam scanning subsystem and the surface ofthe workpiece. The center C of the inspection stage B is referred toherein as the “stage center of rotation.” In the presently preferredembodiments and methods as disclosed herein, a “target spot” TScorresponds to the center of scan position of the output scanner beam.All collectors point to or are configured to receive light emanatingfrom this target spot TS. (The stage center of rotation C is located atthe target spot TS when the center of the wafer is being scanned. Duringthe spiral scan of the wafer, the spiral scan being described in moredetail below, the target spot TS will move further away from the stagecenter of rotation C.) After the incident beam is reflected from theworkpiece surface, it propagates along a light channel axis LC. Theincident beam vector IB and the light channel axis LC define a plane ofincidence PI. A normal plane NP is perpendicular to the base plane B andthe plane of incidence PI. A vector normal N, corresponding to thez-axis, which is perpendicular to the base plane B and which is in theplane of incidence PI, goes through the target spot TS. In addition, thecenter collector axis is on the vector normal N, as will be describedmore fully herein below.

One may construct a hemisphere above the base plane, having a center atthe target spot TS and having a radius approximately equal to thedistance from the stage center of rotation C to the beam scanningsubsystem output, or the collectors as described herein below. Thishemisphere may be bisected into a back quartersphere BQ and a frontquartersphere FQ. The back quartersphere BQ lies between the base planeB and the normal plane NP and contains the incident beam along theincident beam vector IB. The front quartersphere FQ lies between thebase plane B and the normal plane NP, and contains the light channelaxis LC.

Wafers are inserted into inspection zone IZ for inspection and retrievedfrom inspection zone IZ after inspection using the wafer handlingsubsystems 32 and 44. In semiconductor inspection applications andothers as well, the handling of the wafers within the housing preferablyis done automatically, without contact by human hands, to avoid damagingor impairing the surface, e.g., with smudges, scratches, etc. Waferhandling subsystems 32 and 44 provide a plurality of wafers to beinspected. This may be done sequentially or, for system configurationsdesigned to inspect multiple wafers simultaneously, it may providemultiple wafers in parallel. Robotic wafer handling subsystem 44 placesthe wafer or wafers on an inspection stage or table 9 within theinspection zone IZ of housing 42. The robotic wafer handling subsystems32 and 44 may comprise commercially available versions known in theindustry. In the presently preferred embodiments, the robotic waferhandling subsystem 44 comprises a FX3000/2 robotic wafer handlingsubsystem, from Brooks Automation, Inc. (Chelmsford, Mass.). It uses oneor more cassettes, with each cassette holding multiple workpieces (up toten wafers). After placement on the inspection table 9, the wafer isautomatically aligned according to alignment techniques known to thoseof ordinary skill in the art.

System 10 comprises a base 11 that serves as a physical or mechanicalsupport for other components of the system. As implemented in system 10,the base 11 comprises an optics base plate 60 fixedly mounted withininspection zone IZ of housing 42. FIG. 10 provides a perspective view ofbase plate 60. FIGS. 8 and 9 illustrate its positioning and arrangementin system 10. FIG. 21 is a perspective view of the base for the systemof FIG. 1, with the collection and detection subsystem module attached.Base plate 60 in this embodiment is fabricated of black anodizedaluminum. Its surface is coated with a light absorbing coating ortreatment to eliminate or greatly reduce its optical reflectivity. Baseplate 60 includes three kinematic interface points 62 for mounting tothe vibration isolation module, or VIM 40 . The VIM 40, which holds themotorized γ-θ stage 44, rests on isolation mounts 64 to preventvibration from disturbing the light channel signal. Base plate 60 alsohas vacuum lines 64 to remove particles that may be produced by themotorized assemblies located throughout the base plate 60, andpressurized air line port to connect the pneumatic ports 162 forsupplying air pressure to drive the drive shafts 154 of the the AODvariable speed assembly 104, described below.

The workpiece W provided for inspection is held in positionapproximately 1 inch below base plate 60. Base plate 60 includes anaperture 66 approximately in its center and arranged to provide aviewpoint through which the workpiece is viewable. Thus, the workpiece Wresides below aperture 66 during the inspection operations.

Modular Surface Inspection System

It has been noted herein above that, in accordance with an aspect of theinvention, a modular surface inspection system is provided. Preferredsystems according to this aspect of the invention comprise anillumination subsystem 13 having a beam source subsystem 6 forprojecting a beam and a beam scanning subsystem 8 for receiving theincident beam from the beam source subsystem and scanning the incidentbeam on the surface of the workpiece, a workpiece movement subsystem 15that moves the surface of the workpiece relative to the incident beam,an optical collection and detection subsystem 7 that collects thereflected beam and photons scattered from the surface of the workpieceand generates signals in response thereto, and a processing subsystem 19operatively coupled to the collection and detection subsystem 7 forprocessing the signals. Any one or combination of these components maybe modular, each may comprise a field replaceable unit 811. A blockdiagram illustrating the use of field replaceable units 811 is shown inFIG. 74. For example, the beam source subsystem 6 preferably comprises afield replaceable beam source module 70 (also known as laser fieldreplaceable unit 70 or LFRU 70). Further, the beam scanning subsystem 8preferably comprises a field replaceable beam scanning module 92 (alsoknown as AOD field replaceable unit 92 or AFRU 92). In addition, thecollection and detection subsystem 7 preferably comprises a fieldreplaceable collection and detection assembly 200 (also known as acollector-detector field replaceable unit or “DFRU” 200). This modulardesign enables each such component to be assembled during originalmanufacture, or to be maintained or repaired, efficiently and costeffectively. This is particularly necessary in applications, such assemiconductor-related inspection applications, wherein it is importantto minimize system downtime and to maintain critical optical componentalignments in a clean or otherwise controlled environment. Semiconductorwafer inspection systems, for example, typically must operate in cleanrooms. The use of pre-aligned modular components enables the inspectionsystems to be serviced or repaired while being maintained in these cleanor otherwise controlled environments. In addition to their systemconfigurations, each of the modular components as disclosed hereincomprises separate aspects of the invention.

Beam Source Subsystem

The beam source subsystem 6 projects the beam used to illuminate thesurface of the workpiece. The light propagating from the surface, bothspecular and scattered, is then used to characterize or otherwiseprovide useful information about the workpiece surface. The beam sourcesubsystem 6 in this preferred embodiment is modular, and comprises afield replaceable beam source module 70, also known as laser fieldreplaceable unit 70 or LFRU 70. Beam source module 70 is shown inexploded view in FIG. 11, and in assembled state with respect to thebase plate 60 in FIG. 12.

Beam source module 70 comprises a beam source that projects an incidentbeam toward the surface S of the workpiece. Beam source module 70 has abeam source that preferably comprises a laser 72 that projects a beamhaving the desired quality and optical properties for the application athand. The specific characteristics of the laser and the beam it projectsmay vary from application to application, and are based on a number offactors. In applications involving inspection of semiconductor wafers,suitable lasers comprise Argon lasers having a wavelength of about 488nm, semiconductor laser diodes, at several wavelengths (e.g. GaN (405nm), AlGaInP (635 nm-670 nm), and AlGaAs in the 780-860 nm range). Otherlasers include diode-pumped laser such as frequency doubled Nd:YVO4,Nd:YAG, and Nd:YLF (532 nm) and quasi-CW diode pumped UV lasers (355nm). The laser 72 may project a beam that is monochromatic, or whichincludes a plurality of frequencies, etc., depending upon the specificapplication, the desired surface features to be measured, etc.

As implemented in this embodiment, the beam source of beam source module70 comprises a frequency-doubled Nd:YVO4 laser (Spectra Physics MG-532C)operating at 532 nm frequency. The beam comprises a substantiallymonochromatic beam having approximately a 532 nm frequency. The beam hasa beam size at the laser output of 2 mm (full width at 1/e² level). Thebeam is outputted from laser 72 with a power of about 1-2 watts.

Beam source module 70 also comprises a beam source module housing 74that provides structural support for other components of the beam sourcemodule 70. In the. presently preferred embodiment, beam source modulehousing 74 comprises a beam source module base plate 76 upon which laser72 is fixedly disposed. Base plate 76 is constructed of black anodizedaluminum.

Beam source module 70 also comprises laser unit optics 78 for receivingthe beam outputted by the laser 72 and directing it to an appropriatepointing angle and pointing position. With reference to FIG. 13, theoutput of laser 72 passes through a laser shutter 80, provided as asafety mechanism, through a pair of turn mirrors 82 and 84, also knownas turning mirrors or fold mirrors, and through an output aperture 86. Aset of baffles 88 is disposed in the beam path between the turningmirrors 82 and 84 for limiting light that is not contained within themain beam. The alignment of the laser output, the turning mirrors 82 and84 and the output aperture 86 are such that the beam is projected fromthe output substantially at a precise pointing angle and pointingposition. The pointing angle preferably is within 10 to 50 micro radiansof the desired or ideal pointing angle that corresponds to placing thebeam at a desired spot position and angle at the acousto-optic device100, (also known as “acousto-optic deflector”, “AO deflector” or “AOD”and described more fully herein below). The diffracted beam from the AOD100 defines the spot position at the surface of the workpiece W.

Beam source module 70 further includes mounting means for mounting andfixing the beam source module housing 74 relative to the beam scanningmodule base 90, described in more detail below. This mounting meanspreferably fixes the position of the beam source, and more particularlythe beam projected from the output, relative to the base plate 60 sothat, after the beam passes through the AOD 100, the beam is projectedby a pointing angle to a pointing position that is within about 10 toabout 50 micro radians of the desired pointing angle into the AOD. Thediffracted beam from the AOD 100 defines a target spot TS correspondingto a desired spot on the surface of the workpiece. The purpose of the“desired spot” and “desired angle” is to set and fix a point at whichthe laser beam is directed so that, when the system 10 is assembled anda workpiece W is under inspection, the beam is directed to the desiredscanning location on the surface of the workpiece W. The “pointingposition” refers to the location of the beam when it is pointed at thedesired spot TS. The beam source module 70 in this modular embodiment isdesigned to be substantially automatically aligned when placed onto thebase plate 60, so that little or no additional alignment is requiredafter placing the beam source module housing 74 in position. The beamsource module housing 74 may be mounted directly and fixedly on the beamscanning module base 90. Alternatively, the beam source module housing74 may be fixed relative to the base 90 by other means, for example, bymounting it to another component that in turn is mounted to the base 90.In the presently preferred embodiment according to this aspect of theinvention, as shown particularly in FIG. 11, the beam source module baseplate 76 is mounted to a beam scanning module 92, which is a componentof the beam scanning subsystem 8, which in turn is directly mounted tothe base plate 60. This will be explained more fully below.

The mounting means for the beam source module housing 74 in accordancewith this embodiment comprises a plurality of holes or pinholes 94located in the housing 74, preferably in the bottom portion of laserunit base plate 76, designed, sized and configured to receive acorresponding plurality of pins or posts 96 disposed in or on anothercomponent to which the beam scanning module 92 is to be mounted, such asthe beam scanning module base plate 90, so that the pins or posts fitsecurely into pinholes 94. Similarly, the mounting means may comprise aplurality of pins fixedly located in the beam source module housing 74,e.g., in beam source module base plate 76, and projecting outwardly fromit that would mate to a corresponding plurality of holes located in thebase plate 90 or other component to which the beam source module housing74 is to be affixed. In system 10, the mounting means comprises theplurality of holes 94, as shown in FIG. 11, disposed in the bottomportion of laser unit base plate 76 and configured to mate with thecorresponding plurality of pins 96, as shown in FIG. 14, located on theupper surface or portion of the beam scanning module 92, morespecifically beam scanning module base plate 90. The beam source modulehousing 74 is detachably locked into position using socket head capscrews (SHCS)(not shown).

This modular beam source subsystem design provides the beam sourcesubsystem in a self-contained and pre-aligned unit that is modular andfield replaceable. By providing the modular mounting capability and beampre-alignment, this design facilitates the ready installation orreplacement of the unit on the system, quickly, efficiently, and withoutthe need for substantial additional adjustments, alignments, etc.commonly required in prior known systems. A separate alignment fixturemay be used to ensure that all of the laser source assemblies areco-aligned to ensure that no alignment is necessary in the field.

Beam Scanning Subsystem

System 10 also includes means for receiving the incident beam andscanning the incident beam on the surface of the workpiece. In thispresently preferred embodiment, the beam scanning means comprises a beamscanning subsystem 8, which, in this preferred embodiment, is modularand, in this illustrative modular system, comprises a field-replaceablebeam scanning module 92 (also known herein as AOD field replaceable unit92 or AFRU 92).

The beam scanning module 92 receives the beam from the beam sourcemodule 70 and scans it on the surface S of the workpiece W in desiredfashion. As noted herein above, a variety of different scan patterns areavailable, and the one used in a particular instance may vary fromapplication to application.

Beam scanning module 92 is shown in perspective and exploded viewrelative to base plate 60 and beam source module 70 in FIG. 11. It isshown in its assembled stated mounted to base plate 60 in FIG. 12. A topview of beam scanning module 92, shown separately, is provided in FIG.14.

The beam scanning subsystem 8 comprises means 198, mounted in a fixedposition relative to the housing, for scanning the beam on the surface Sof the workpiece W, also known as beam scanning means 198 and showngenerally in FIG. 15. A number of alternative scanning means may be usedto scan the beam in desired fashion. Examples include acousto-opticdeflectors (AODs), rotating mirrors, and the like. In the presentlypreferred embodiments and method implementations, the beam scanningmeans 198 comprises an acousto-optic deflector (AOD) 100, showngenerally in FIG. 16.

The acousto-optic deflector 100 may be any acousto-optic deflector,including but not limited to those commercially available, that iscapable of or suited for the beam and beam source to be used, thedesired scanning parameters (e.g., beam and spot size, scan pattern,scan line dimensions, etc.), and other design requirements andconstraints. The AOD 100 according to the presently preferred embodimentand method implementations comprises the ISOMET Model OAD-948R (488 nm)or, alternatively, the ISOMET OAD-971 (532 nm), both of which areavailable from Isomet Corporation of Springfield, Va.

The beam scanning module 92 also comprises a beam scanning modulehousing 98 fixedly coupled to or integral with base plate 90 forsupporting the beam scanning means 198. Housing 98, shown in FIG. 14,comprises an AOD assembly 102 that houses the AOD 100, which comprisesan AOD crystal 112 and related components. AOD assembly 102 is mountedto or fixed to beam scanning module base plate 90, or with which itforms an integral part.

Variable Speed AOD

Wit reference to FIG. 15, AOD assembly 102 comprises a variable speedassembly 104 for selecting or varying the scanning speed of the AOD 100while maintaining good beam quality. Variable speed assembly 104comprises a motor drive assembly 106 (if using an electric motor) or anair drive cylinder 108 (if driven pneumatically), and at least one driveshaft 154. Referring to FIG. 16, which shows a cutaway view of a portionof the AOD assembly 102, and FIG. 17, which shows a top view, AODassembly 102 includes a beam input or aperture 110 at which AOD assembly102 receives the incident beam from beam source module 70. AOD assembly102 also includes an AOD crystal 112 positioned in the optical path ofthe beam. An RF drive system (not shown) is provided to scan the outputangle of the diffracted beam emitted by the AOD crystal. In operation,the RF drive system provides an acoustic signal across AOD crystal 112,which causes the refractive index of the crystal to vary across itsface. As the frequency of the RF drive system is changed, the lightpassing through the crystal interacts with the acoustic beam and isdiffracted with an angle that is directly related to the frequency ofthe RF drive. This incident light also is split into. separate beams, sothat the zeroth order beam passes straight through the crystal 112, butother orders, e.g., the 1st order, the—1st order, etc. are deflected.These orders are shown in FIG. 17 at 115. In the presently preferredembodiment, i.e., system 10, the drive signal is varied in frequency inproportion to a sawtooth voltage signal, so the beam is deflected in theplane of the page for FIG. 17. Stops or baffles 114 in the form of wellpolished black glass are provided within AOD assembly 102 for blockingorders other than the +1^(st) order, and for limiting and clipping thescan of the beam. These stops 114 are oriented with respect to the beamat the Brewster's angle to maximize the absorption of the unwanteddiffracted beams from the AOD crystal 112. An adjustable aperture 116 islocated in the optical path of the beam. A wave plate 118 is disposed inthe optical path to rotate the output polarization of the light. Atelecentric lens 120 is positioned in the optical path after the waveplate 118. This lens 120 focuses the beam down to a spot at the surfaceunder test or inspection. The spot size in this preferred embodiment isnominally 50 microns in the in-scan direction and 120 microns in thecross-scan direction.

AOD SNR Improvement

The AOD assembly 102 also includes a beam scan absorbing system 24 forabsorbing light that is not collimated in the beam. In this embodimentthis beam scan absorbing system 24 comprises a series of apertures,baffles and threads, including optical baffling or optical threads 122located in the snout 124 of the AOD assembly 102 near its output 126.The beam is output from AOD assembly 102 at a beam output aperture 126.

Beam Scanning Module 92 Mounting

The beam scanning module 92 further includes beam scanning modulemounting means 196 for fixedly mounting the beam scanning module housing98 relative to the base 11 so that the beam is projected at a pointingangle to the pointing position. As was noted in connection with the beamsource module 70, it is desirable for the beam scanning module 92 to beeasily mounted, pre-aligned, and to require a minimum of alignment orother adjustment to install it onto the system. Proper operation of alaser-based surface inspection system requires AOD alignment tolerancesto be quite tight. It can be difficult to obtain the requireddiffraction efficiency and power uniformity necessary for proper AODoperation when aligning the AOD 100 during system assembly. Replacingthe AOD assembly 102 in system 10, as is occasionally necessary duringservicing of surface inspection systems, requires duplicating the AODalignment in order to obtain the same diffraction efficiency and poweruniformity. Re-alignment could therefore result in loss of systemsensitivity. Obtaining correct alignment, while critical, is made evenmore difficult when the AOD 100 must be replaced in the field. It isdifficult to enable field replaceability of the AOD while ensuring thatthe laser beam will be aligned with respect to the AOD within such tighttolerances. The beam scanning module 92 according to the presentlypreferred embodiments therefore comprises a modular and fieldreplaceable unit.

In this specific yet illustrative embodiment, the beam scanning modulemounting means 196 comprises a plurality of pins 96 in the beam scanningmodule base 90 that mate a corresponding plurality of pinholes 94 in thebeam source module base plate 76. Alternatively, or in combination, theplurality of pinholes could be in another system component to which thebeam scanning module 92 and the beam source module 70 are to be affixed.Also alternatively or in combination, the mounting means may comprise aplurality of pinholes in the surface of beam scanning module base 90that would mate to a corresponding plurality of pins in the bottomsurface or portion of the beam source module base plate 76.

As noted above, beam scanning base plate 90 also comprises means 196 formounting the beam scanning module 92 to base plate 60 or other systemcomponent through which the beam scanning module 92 is to be affixed tobase plate 60. In this presently preferred embodiment, the mountingmeans 196 comprise a plurality of pins 128 on the bottom surface of beamscanning module base plate 90 that mate with pinholes 130 in base plate60. Alternatively or in combination, the mounting means 196 may comprisea plurality of pinholes in the bottom surface of base plate 90 thatwould mate to a corresponding plurality of pins in the top surface orportion of base plate 60.

Variable Speed AOD, Contd.

In accordance with another aspect of the invention, a variable scanningspeed acousto-optical deflector assembly 194 is provided. This assemblymay be provided separately, or it may comprise a component in a surfaceinspection system. This assembly comprises an AOD 100, means 190operatively coupled to the AOD 100 for varying the scan speed at whichthe AOD 100 deflects a beam passing through the AOD 100 (the means 196also known as the AOD scan speed varying means 196), and beamastigmatism compensating means 160 for compensating for astigmatism ofthe laser beam associated with the variation of scan speed.

Beam scanning module 92 as described herein is designed to make betteruse of the relatively high detection-throughput capability of system 10over prior known systems. “Detection-throughput” is analogous to the“gain-bandwidth product” known by those skilled in the art of electricalengineering. Detection-throughput determines how many wafers per hour ascanner can scan at a given detection sensitivity performance level.Alternately, it is the ultimate detection sensitivity the system canachieve at a given throughput level. As the detection-throughputcapability increases, the wafer can detect smaller defects at higherthroughput, thereby lowering the cost of ownership. Methods forincreasing the overall detection-throughput include increasing the laserpower, improving the collection efficiency of the detection collectors,and increasing the quantum efficiency of the detectors.

The ability of a beam scanning subsystem 8 to flexibly trade betweendetection sensitivity and throughput can be and often is very important.In some prior systems, the scan speed is fixed, and therefore thesensitivity that can be achieved also is fixed. If a system could scanmore slowly, the system could effectively integrate more photons,thereby reducing the shot noise levels (described in more detail below)and improving the sensitivity of the system to smaller defects. If asystem could be scanned more quickly, the throughput could be increasedbeyond its current speed, reducing the cost of ownership of the tool atthe expense of defect sensitivity. By enabling such systems to scanmultiple speeds, the user can advantageously trade off throughput forsensitivity in a flexible manner. By offering multiple or even manyeffective speeds, the user can choose the speed that is right for theirparticular process.

Multiple speed operation in surface inspection systems having shortscanning capability can be achieved by two methods: 1) changing thecross-scan speed or the cross-scan pitch (slower stage rotational rate)in cooperation with cross-scan filtering to match the filtercoefficients of the cross-scan pulse signal shape (see, e.g., U.S. Pat.No. 6,529,270, which is hereby incorporated by reference), and 2)changing the in-scan speed and adjusting the in-scan filter for propermatched filtering. Preferred systems and methods according to thisaspect of the invention use both methods to provide a series ofselectable scan speeds.

Method 1 does not require any changes to the optical design while Method2 often will. Method 2 requires changing the AOD modulation frequencyper unit time, requiring the AOD frequency chirp range (or scan length)to be reduced while holding the AOD scan time invariant in order to scanthe spot more slowly during the same in-scan time base. The in-scanspeed is controlled by the total change in the AOD modulation frequencyper unit time. If the AOD scan length is reduced, the effective internallens focal length in the AOD will also change, requiring the cylinderlens focal length to change in order to compensate for the new AODin-scan focal length. If the active compensation is incorrect (i.e., thecylinder lens focal length does not match that of the AOD lens), thein-scan spot size will be too large at the wafer plane, and theeffective sensitivity of the scanner will be reduced.

Reducing the AOD scan speed provides an improvement in particle diametersensitivity, which is the result of quantum mechanical shot noise. Itmay be quantified as[(√R)^(1/6) ]=, d _(f) /d _(s), where:

R is the ratio of a full scan speed to a slower scan speed;

d_(f) is the diameter of the particle that is discemable at full speed;and

d_(s) is the diameter of the particle that is discemable at the slowerspeed.

By using a combination of both Methods 1 and 2, a large selection ofscan speeds can be chosen along the detection-throughput curve. Forexample, in an illustrative but not necessarily preferred embodiment,the AOD scan rate is 20 microseconds per AOD scan, with 16 microsecondsfor the AOD scan and 4 microseconds for the fly-back to the AOD scanstart position, nominally 4, 3, 2, and 1 mm AOD scan lengths areselectable, and 23, 11, 6, and 3 micron AOD cross scan pitches areselectable. If the cross scan filter can support 23, 11, 6, and 3 microncross scan pitches, and the in-scan beam scanning subsystem 8 cansupport 4, 3, 2, and 1 mm AOD scan lengths, the system 10 can operate ata total of 4×4=16 scan speeds. The 3 micron cross scan pitch, whenutilized with the 1 mm AOD scan length, can provide the best sensitivityat the lowest throughput, while the 23 micron pitch and 4 mm scan lengthwould provide the highest throughput. By providing 16 or more scanspeeds along the detection-throughput curve, the user can select theoptimal speed/sensitivity setting for their particular processes.

In the presently preferred yet merely illustrative embodiment , once anAOD scan speed is selected, it is maintained throughout the wafer scan.The setting does not vary within a given AOD scan.

A variable speed acousto-optic deflector assembly 194 according to apresently preferred yet merely illustrative embodiment of this aspect ofthe invention is shown in FIGS. 15-17 and in block diagram form in FIG.84. The acousto-optical deflector according to this aspect of theinvention may comprise any AOD suitable for the application and capableof meeting the technical requirements at hand. The presently preferredAOD is AOD 100 of AOD assembly 102.

The variable speed AOD assembly 194 also comprises means 190 operativelycoupled to the AOD 100 for varying the AOD scan speed at which theacousto-optical deflector scans a beam passing through it. The specificmeans 190 that may be used to perform this task will depend upon thespecific AOD used and in some cases other factors as well. It normallywill involve drive electronics used to drive the AOD, such as thatcommercially available from AOD suppliers.

In the presently preferred embodiment, the AOD scan speed varying means190 comprises digital drive circuitry 180 comprising a digital voltagecontrolled oscillator (“DVCO”) 182, such as IDDS-1-SE Direct DigitalVCO, and radio frequency (“RF”) power amplifier 184, such asIA-100-3-826 RF Power Amplifier, both commercially available from ISOMETof Springfield, Va. It also comprises a gauge synchronization board 186(also known herein as gauge synchronization control 186) withsynchronization signals that trigger the DVCO 182 to initiate AOD scans.The digital drive circuitry 180 includes a control 188 to selectivelyvary the scan speed of the AOD, and/or to select discrete scan speeds.The AOD scan speed varying means 190 has software controls 181 and stageservo controls 189 to accomplish the speed changes necessary whenchanging the cross-scan speed or the cross-scan pitch.

The AOD scan speed varying means 190 also has in-scan and cross-scanfiltering circuitry 183 comprising electronic circuitry 185, which maycomprise digital circuitry. However, in the presently preferred yetmerely illustrative embodiment, the filtering circuitry 185 comprises ananalog low-pass filter 187 with an impulse function which matches thepulse width produced by the AOD scan to maximize signal to noise ratioin the in-scan direction.

Beam Compensating Means

The variable scan speed AOD assembly 194 also comprises has a beamcompensating lens 150 that operates to produce a focal length differencebetween in-scan and cross-scan direction and a beam astigmatismcompensating means 160 for varying the focal length difference in orderto compensate for astigmatism of the beam associated with the variationin scan speed. As the scan speed of the AOD 100 is changed, theastigmatism of the beam also changes. This astigmatic effect usually isdisadvantageous, for example, in that it spreads and defocuses the beam.The beam astigmatism compensating means 160 is used to compensate forthis astigmatism so that its adverse effects can be offset or eliminatedand the desired beam geometry can be obtained.

The focal length (L) for a selected AOD in-scan speed is a function ofthe laser beam size, the frequency shift across half of the laser beam,and the laser wavelength. It is calculated as follows:

The AOD Sweep rate (R) is calculated as:

$\begin{matrix}{{R = {\frac{\Delta\; F}{P}\left( {{Hz}\text{/}s} \right)}},} & \;\end{matrix}$where

-   -   ΔF=Total frequency difference between lowest and highest        frequencies during AOD sweep; and    -   P=Sweep Period, defined to be the total time required for the        AOD to sweep from lowest to highest frequency.        The time across beam (T) is calculated as:

${T = \frac{B}{S}};$where

-   -   B=Beam size, and    -   S=Speed of sound in crystal.        The frequency shift across half of the beam (H) may be        calculated as        H=R*T/2.        The focal length L for a selected AOD in-scan speed may then be        calculated as

${L = \frac{B}{2*\left\lbrack {{Tan}\left( {H*{W/S}} \right)} \right\rbrack}},$where

B=beam size;

H=frequency shift across half of beam;

W=laser wavelength; and

S=Speed of sound in crystal.

As noted above, the telecentric lens 120 is positioned in the opticalpath after the wave plate 118 and before the optical threads 122 nearthe optical threads 122 to convert the angular scan to a spot positionscan at the workpiece surface, while simultaneously focusing the beam atthe workpiece. The telecentric lens 120, when it is properly matched tothe effective lensing effect in the AOD (lens 150), ensures that thein-scan and cross-scan waists are located at the same position along theoptical axis. However, as the AOD scan speed varying means 190 variesscan speed, the focal length (L) changes and. Therefore, the effectivelensing effect in the AOD 100 changes in response to the AOD scan speed,introducing an astigmatism. The beam astigmatism compensating means 160performs an astigmatic correction. The beam astigmatism compensatingmeans 160 operates to modify the effective lensing effect in the AOD 100in order to allow the telecentric lens 120 to maintain focus of the beamat the workpiece W onto a spot position at the workpiece surface atvarying scan speeds.

The beam astigmatism compensating means 160 may comprise any means inwhich the focal length of a lens system may be varied in response to achange in the index of refraction or lens surface curvature. Forexample, the beam astigmatism compensating means 160 may comprise aliquid lens, in which the surface curvature is changeable, or preferablya lens system 192 comprising a plurality of lenses in which the focallengths of the respective lenses differ from one another and areselected to appropriately compensate for the beam deformation at each ofthe respective desired scan speeds. Cylindrical lenses are particularlypreferred. The beam astigmatism compensating means 160 also preferablycomprises a lens positioning device 172 operatively coupled to theplurality of lenses. The lens positioning device 172 is used to positiona selected one of the lenses in the lens system 192 in the beam at theoutput of the AOD 100, in the optical path of the beam. Each lens in thelens system 192 is designed to provide the desired beam compensation fora given AOD scan speed and provides a unique amount of compensationrelative to that of others of the lenses in the lens system 192. Thelens positioning device 172 is used to alternately position the lensthat corresponds to the selected scan speed into the beam path at ornear the AOD output. When the AOD scan speed is changed, the currentlens is moved away from this position, and another one of the lenses,this one being compatible with the newly selected AOD scan speed, ismoved into position at or near the AOD output and in the beam path.

In the presently preferred embodiments, and with reference to FIGS.15-17, the beam astigmatism compensating means 160 comprises a lenssystem 192, having two cylindrical lenses 150 a, 150 b housed in a lenshousing 152 located between the AOD crystal 112 and near the opticalthreads within AOD assembly 102, and a lens positioning device 172 forcontrolling the positioning of the lenses with a sliding plate 158 formoving the variable speed assembly cylindrical lens 150A or 150B intoposition.

The lens positioning device 172 comprises a variable speed assembly 104that uses a motor drive assembly 106 comprising an electric motor, notshown, or, alternatively, an air drive cylinder 108, connected to driveshafts 154 that rigidly connect the motor drive assembly 106 to thecylinder lens housing 152. Motor drive assembly 106 may comprise anydrive assembly to move the lenses, e.g., such as those means notedherein above. In the presently preferred yet merely illustrativeembodiment, the motor drive assembly 106 operates pneumatically and thusincludes a pneumatic pressure source 156 and pneumatic ports 162 forsupplying air pressure to drive the drive shafts 154. A pair of springs164 is positioned on drive shafts 154 to prevent the lens assembly 192from being overdriven.

When AOD assembly 102 and its associated drive circuitry are set to scanat a first scan speed, variable speed assembly 104, including motordrive assembly 106, are used to position lens 150 a, 150 b in the beampath (the up position for variable speed assembly 104 as shown in FIG.16). Lens 150 a provides the amount of compensation appropriate tooffset the astigmatism associated with the first scan speed. When AODand its associated drive circuitry are set to scan at a second scanspeed different from the first scan speed, in this case, slower than thefirst scan speed, the variable speed assembly moves the drive shaftsdown to position lens in the beam path. Lens 150 b is designed toprovide the appropriate amount of compensation to offset the astigmatismassociated with the second scan speed.

Another embodiment of a variable speed AOD in accordance with thisaspect of the invention is shown in FIG. 18. In it, the beam astigmatismcompensating means 160 comprises a lens housing 170 with a rotatingcarousel 171 that contains multiple lenses, preferably cylindricallenses, 170 a, 170 b and 170 c. Housing 170 selectively moves one of theplurality of lenses 170 a, 170 b, 170 c into the beam path by rotatingthe carousel 171.

In each of these embodiments, the lenses preferably but optionally arepositioned immediately adjacent to the acousto-optical deflector 100.

Scan Repetition Mode and Station

In accordance with another aspect of the invention, a method forinspecting the surface of a workpiece, in which an incident beam isprojected toward the surface of the workpiece, and the surface of theworkpiece is scanned to generate a scan output representative of theeffects on the surface of the incident beam, comprises a method 231 forrepeatedly scanning a selected scan region of a workpiece. As shown inFIG. 82, the scan repetition method 231 comprises the step 215 of movinga workpiece relative to the incident scanned beam and the step 232 ofrepeatedly scanning a selected scan region of the workpiece to produce aset of repeated scans.

In a further embodiment, the selected scan region has a plurality ofsample locations, and the step 231 of repeatedly scanning a selectedscan region of the workpiece further comprises a step 233 of generatinga repeated scan output comprising, for each of said sample locations,generating a set of signals associated with the sample location over theset of repeated scans.

In a multi-collector surface inspection system such as system 10, asurface scan produces, from each collector, a signal associated witheach sample location, and the step 233 of generating a scan outputcomprises, for each of said sample locations, generating a set ofsignals associated with the sample location, from each collector andover the set of repeated scans.

In another embodiment, the step 232 of repeatedly scanning a selectedscan region of the workpiece further comprises the step 235 of selectinga quantity of scan repetitions, for defining the number of scans to berun on a selected scan region of the surface, and a step 234 ofselecting a scan region for defining a region of the workpiece to bescanned.

The resulting scan repetitions may be used to increase the Signal toNoise Ratio (SNR) of the selected scan region and thus reveal greaterdetails of the surface under consideration. SNR may be improved byaggregating the output of a set of scans that are repeated on a selectedregion. Therefore, in another embodiment, the scan repetition method 231further comprises the step 236 of aggregating the scan output. In oneembodiment, the step of aggregating comprises the step 237 of averagingthe scan output, for example finding the arithmetic mean of the scanoutput. In a more preferred embodiment, the step 237 of averaging thescan output comprises the step 238 of frame averaging the scanrepetitions.

Frame averaging is a mathematical process in which several frames ofidentical scenes are coincided to produce an increase in detail andthereby resolution of the scene. In the context of scan repetition in asurface inspection system such as system 10, frame averaging comprisesaveraging each of the sample signals associated with a sample locationwithin a selected scan region over the set of repeated scans. In thecontext of scan repetition in a multi-collector surface inspectionsystem such as system 10, frame averaging comprises, for a samplelocation, averaging each of the sample signals associated therewith fromeach collector. A discussion of frame averaging may be found at TheImage Processing Handbook, 3rd ed., John C. Russ (CRC Press IEEE Press1998). Frame averaging minimizes shot noise and enhances the signal frompersistent scatter sources by lowering the signal value of shot noise inthose locations where shot noise is present.

While the random nature of shot noise results in random signals during ascan, real surface scatter sources may produce a signal at the samelocation for each collector. Further, real surface scatter sources mayproduce a signal at the same location for each collector every time thata scan is repeated. When signals which are the output from a set ofscans that are repeated on a selected scan region are averaged, signalsfrom a real surface scatter source will more likely produce a higheraverage signal. However, shot noise signals from the region, which bydefinition generally do not repeat in the same locations, will result ina lower average signal. Thus the SNR will be improved by frame averaginga set of scans that are repeated on a selected scan region.

In accordance with another aspect of the invention, a scan repetitionsystem 38 is provided. The scan repetition system 38, which may beprovided separately, or which may comprise a component in a surfaceinspection system, comprises a workpiece movement subsystem 15 formovement of the wafer relative to an incident scanned beam and a system31 operatively coupled for repeatedly scanning a scan region of theworkpiece. The scan repetition system 38 is shown in FIG. 83.

In a further embodiment, the selected scan region has a plurality ofsample locations, and the system 31 operatively coupled for repeatedlyscanning a scan region of the workpiece further comprises a system 33for generating a repeated scan output, which generates, for each of saidsample locations, a set of signals associated with the sample locationover the set of repeated scans. In a multi-collector surface inspectionsystem, the system 33 for generating a repeated scan output generates,for each of said sample locations, a set of signals associated therewithfrom each collector and over the set of repeated scans.

In a further embodiment, the scan repetition system 38 further comprisesa scan repetition quantity selector 35, for defining the number of scansto be run on a surface, and a scan region selector 45 for defining aregion of the workpiece to be scanned.

The scan repetition system 38 may comprise any scan repetition systemsuitable for the application and capable of meeting the technicalrequirements at hand. The scan repetition system 38 preferably comprisessoftware controls 37 and stage servo controls 39. In addition, each ofthe specific implementations of the system 31 for repeatedly scanning ascan region of the workpiece scan, the repetition quantity selector 35,and the scan region selector 45 will depend upon the specific workpiecemovement subsystem 15 used and in some cases other factors as well. Inthe presently preferred embodiment, the repetition scan system 31 andits systems 33, 35, 45, 36, 47, 49 are operable using software controls37 and stage servo controls 39.

In a further embodiment, the scan repetition system 38 further comprisesa scan output aggregator 36 to aggregate the output of a set of scansthat are repeated on a selected region. In one embodiment, the scanoutput aggregator comprises a system 47 for averaging scan output. In amore preferred embodiment, the system 47 for averaging output comprisesa system 49 for frame averaging for averaging each of the sample signalsof each collector from each of the sample locations within therepetition region.

Using the scan repetition system 38 and scan repetition method 231, thesurface inspection system may scan an entire wafer and then makemultiple scans of sub-regions of the wafer wherever there are defects ofinterest. Use of the scan repetition system 38 and method 231 can allowdetection of defects with <30 nm PSL equivalent sizes.

Optical Collection and Detection Subsystem

In accordance with another aspect of the invention, an opticalcollection and detection subsystem 7 is provided. The optical collectionand detection subsystem 7 may be provided as an independent assembly, orit may be incorporated into a surface inspection system, for example,such as system 10. It comprises a collection system 380 and a detectionsystem 480. The collection system 380 comprises components used tocollect the beam portions reflected from the surface of the workpieceand scattered from the surface due to surface roughness, defects in thesurface, and the like. The detection system 480 is operatively coupledto the collection subsystem 380 and works in conjunction with it todetect the collected light and convert it into corresponding signals,e.g., electrical signals, that can be utilized by the processingsubsystem to obtain information pertaining to the surface of theworkpiece.

Architecture

The optical collection and detection subsystem 7 (FIG. 21) in accordancewith the presently preferred embodiment of this aspect of the inventionoperates to collect portions of the incident beam that are scattered andreflected from the surface of the workpiece and generates signals inresponse to them. As implemented in system 10 and shown in FIG. 20, thecollection and detection subsystem 7 comprises an optical collectorsubsystem 380 and a detector subsystem 480, and the signals compriseelectrical signals, each of which having a voltage that is proportionalto the optical power illuminating the detector subsystem 480. Thecollection and detection subsystem 7 in its various implementations asdescribed herein and claimed herein below, comprise additional aspectsof the invention, in the system embodiments as well as separately.

The optical collection and detection subsystem 7 comprises means 250 fordeveloping a light channel, for collecting the beam reflected from thesurface of the workpiece into a light channel, and means 260 fordeveloping a dark channel, for collecting the portions of the beamscattered from the surface into a dark channel collector. The means 260for developing a dark channel further comprises components of theoptical collection and detection subsystem7, described in more detailbelow.

As shown in FIG. 1, the optical collection and detection subsystem 7comprises a series of collection and detection assemblies 200 (alsoknown as collection and detection modules 200), each assembly 200comprising components of the optical collection subsystem 380 and thedetection subsystem 480 and each assembly 200 organized into a collectormodule 300 (also referred to herein as “collector”) for collectingportions of the beam, and a detector module 400 associated therewith.The means 250 for developing a light channel comprises the components ofthe collection and detection assemblies 200 for collecting and detectingthe specular beam and, the means 260 for developing a dark. channelcomprises the components of the collection and detection assemblies 200for collecting and detecting the scattered portions of the beam,

In the illustrative but not necessarily preferred embodiment and asshown in FIGS. 1 and 2, the series of collection and detectionassemblies 200 comprises a front collection and detection module 230, acenter (or central) collection and detection module 220, a pair of wingcollection and detection modules 210A, 210B, and a pair of backcollection and detection modules 240A, 240B. FIG. 19 provides aperspective view of optical collection and detection subsystem 7. FIG.20 shows a side cutaway view of it. FIG. 21 shows the subsystem 7attached to base plate 60. FIG. 22 shows a bottom view of the collectionand detection subsystem 7.

Although all of the collector-detector assemblies 200 need notnecessarily all be of the same design and construction, in thispreferred embodiment each of them has the same basic design, which isillustrated by back collector-detector assembly 240A in FIGS. 23-26.FIG. 23 provides a perspective view of the assembly 240A from a first orfront perspective, FIG. 24 provides a perspective view from a viewopposite the first or front perspective, and FIG. 25 is a side cutawayview.

Referring to FIG. 25, the collector-detector assembly 240A comprises acollector module 300 that includes a collection optics subassembly 390mounted in a barrel housing 394. A variety of lens designs may be used,for example, depending upon the specific application, the budget, etc.In other embodiments, the collector module300 could comprisearrangements other than lens assemblies. For example, mirrors could beused to direct the scatter to a detector. In the illustrative but notnecessarily preferred embodiment, the collection optics subassembly 390comprises collector objective lens optics 392 having aspheric lenses L1,L2. Objective lens optics 392 focuses the incoming beam to a slit 396.Lens L1 collimates the light scattered from the workpiece, while L2focuses the light to the slit 396, which operates as a field stop toabsorb scatter outside of the region being scanned by the laser spot.When the collector objective lens optics 392 comprise aspheric lenses, awide collection angle, such as about a 60 degree total angle) may beachieved while a small image spot Point Spread Function (“PSF”) isproduced at the slit 396. Alternatively, the collector objective lensoptics 392 could comprise doublet lenses.

A detector module 400 is mounted to the collector module 300. Detectormodule 400 includes a detector module barrel housing 494 that mates withcollector module barrel housing 394 adjacent to slit 396 and a relaylens assembly 490. Relay lens assembly 490 comprises a relay opticcollimating lens L3 that is disposed in housing in the beam pathadjacent to slit 396, a relay optic focusing lens L4 that is positionedat the opposite end of housing 494, and a lens L5 (between L4 and thefinal slit 496) that produces the desired spot size-on the photocathodesurface. FIG. 26 is a perspective view of collector module 300 and aportion of the detector module 400, excluding the detection units shownin FIG. 25. As shown in FIG. 26, a slit 496 also is provided in therelay lens assembly 490 near the detection unit.

A first detection unit 492 is mounted to detector barrel housing 494adjacent to the focusing relay optics lens L4. Detection unit 492comprises a detector 497, such as a photo-multiplier tube (“PMT”), suchas the Hamamatsu H6779-20, or an Avalanche Photodiode (APD) Detector(e.g. Advanced Photonix 197-70-74-581), or other type of detector thatis sensitive to receive and detect portions of the light beam passingthrough a lens. A second detection unit 493 is provided at the side ofdetector barrel housing 494. Second detection unit 493 according to thisembodiment is substantially identical to first detection unit 492, andincludes a detector 499 such as the PMT identified above (although it ispermitted in the illustrative embodiment that the PMTs found indetection units 492 or 293 may be different in design, hereinafter a PMTmay be referred to generally as PMT 495). Each of the detectors 497, 499detects a specific polarization orientation. For example, while one PMT495 collects scattered light that is polarized in the “P” orientation,the other PMT 495 collects light in the “S” orientation. This is becauseeach PMT 495 is positioned to collect the “P” and “S” polarized lightthat is emitted by the polarizing beam splitter cube 472 that is locatedin the relay lens assembly 490.

Beam Scanning Subsystem, Contd.

PMT at a Telecentric Plane and Stationary Laser Spot

In accordance with another aspect of the invention, one or more of thedetectors 497, 499 is designed so that the photomultiplier tube 495 orother detection device is located at a telecentric plane or stop 498with respect to the collection optics. This can help to ensure that thelaser spot is stationary on the PMT photocathode surface during the AODscan, or is limited in movement on the detector. This correspondence canhelp to eliminate detector-induced banding effects across the scan. Asimplemented in the presently preferred embodiment, the plane of eachdetector 497, 499 in collector and detector assemblies 200 is located ata telecentric plane 498 with respect to the collection optics 392.Referring to FIG. 25, which shows a back collector-detector assembly240A but is illustrative of the other collector-detector assemblies 200as well, telecentric planes or stop locations 498 are imaged at P1, P2,P3 so as to ensure minimal spot movement at the detector, therebyreducing background signal non-uniformity. Refer to pages 142-143 ofModern Optical Engineering 2^(nd) ed., Warren J. Smith (McGraw-Hill,1990), for a description about telecentric stops.

Optical Collection and Detection Subsystem, Contd.

Variable Polarization

In accordance with yet another aspect of the invention, the collectionand detection assembly 200 comprises a relay assembly 490 (FIG. 72)further comprising a polarizing relay assembly 450 positioned betweenthe collection optics subassembly 390 and the detectors 497, 499. In afurther aspect of the invention, the polarizing relay assembly 450further comprises a variable polarizing assembly 470. This variablepolarizing assembly 470, also known herein as rotational analyzer 470and rotational polarization filter 470, is capable of selectivelypassing solely P polarization, or solely S polarization, or combinationsthereof. Referring to the back collection and detection assembly 240Aillustrated in FIG. 25, a presently preferred variable polarizingassembly 470 according to this aspect of the invention will now bedescribed. Variable polarizing assembly 470 in this embodiment isintegrated into the detector module 400. Assembly 470, in thisembodiment also known herein as dual channel variable rotationalanalyzer assembly 470, comprises a motor-driven rotational polarizeranalyzer 461 (also known as dual detector polarization analyzer 461)having a beamsplitter cube 472 and dual detectors 497, 499. Thepolarizing beamsplitter 472 is fixedly positioned in a chamber 473 ofdetector module 400, in the light scatter path. The beamsplitter 472 ispositioned so that a transmitted portion of the scattered light passesthrough the beamsplitter 472 and impinges upon a first detector 497 infirst detector unit 492 as a flux of photons having a first selectedpolarization, and a reflected portion of the scattered light passesthrough the beamsplitter 472 and impinges on a second detector 499 in asecond detector unit 493 as a flux of photons having a second selectedpolarization, for dual PMT implementations. A rotational mechanism 474,such as motor, rotates the chamber 473 and thus the polarizingbeamsplitter 472 to alter the polarization of the light impinging on thedetection units 492, 493. Second detector 499 is fixed with respect tofirst detector 497, and thus second detector 499 also rotates with theassembly 470. A motor 476 or similar drive mechanism is provided which,upon actuation, causes the chamber 473, including beamsplitter 472, andsecond detector 499 to rotate.

To illustrate the construction and operation of this assembly 470,assume that the incident photons are unpolarized, and that beamsplitter472 is oriented in chamber 473, and chamber 473 is oriented, so thatpolarizing beamsplitter 472 transforms a portion of the unpolarized beaminto P polarized light for transmission to the first detector 497.Simultaneously, polarizing beamsplitter 472 transforms a portion of thephotons S-polarized light for transmission to the second detector 499.If a different polarization mix is desired, motor 476 causes theassembly 470, including chamber 473, polarizing beamsplitter 472, anddetectors 497, 499 to rotate. This causes the polarizing beamsplitter472 to transform the portion of the scattered light impinging on thefirst detector 497 into a first selected mixture of P polarized lightand S polarized light. This also causes the polarizing beamsplitter 472to transform the portion of the scattered light impinging on the seconddetector 499 into a second selected mixture of P polarized light and Spolarized light. Preferably, the polarizing beamsplitter 472 accordingto this aspect of the invention has multiple selectable polarizationsettings, and more preferably are infinitely selectable over a desiredrange.

The motor 476 may causes the assembly to rotate to any desiredpolarization mix, or it may be arranged to step through selectedpolarization mixes. Alternatively, the assembly 470 may have aprogrammed polarization mix mechanism 478, which may be any knowncombination of hardware and software elements, that is arranged toprovide a combination of infinitely selectable mixes and steppedpolarization mixes, with the stepped polarization mixes changeable atthe option of the user.

A variable polarization assembly 470 according to a second preferredembodiment, shown in FIG. 71, comprises an optional motor-drivenrotating carousel polarization analyzer 451 with a rotating carousel 453that contains multiple glass cubes. Carousel 453 selectively moves oneof the plurality of cubes 475, 477, 479 into the beam path. In anillustrative but not necessarily preferred embodiment of the presentinvention, the motor-driven rotating carousel polarization analyzer 451comprises three glass cubes 262: one polarization beamsplitter cube(“PBS”) 475 oriented for local P-polarization, one PBS cube 477 orientedfor S-polarization, and one non-polarizing cube 479 for unpolarizedlight. By using a glass cube 479 for the unpolarized light, theeffective optical path length through the relay lens assembly ismaintained. This is required to maintain the same spot shape at the PMTphotocathode. Incorporating three cubes into the rotating carouselpolarization analyzer 451 simplifies the assembly design, and enablesthe analyzer to change polarization states quickly and accurately. Thisanalyzer 451 can be easily interchangeable with the fixed polarizerrelays. As noted herein, different virtual masks 131, which aredescribed in more detail below, can be switched in and out using thisassembly as well.

Rotational carousel analyzers 451 as described herein can be used toelectronically select each of a plurality of cubes 475, 477, 479, eachof which can utilize a different virtual mask 131 shape and size. Thisenables the detection subsystem 480 to have a refined angular resolvedscatter defect detection capability in a versatile manner by eitherselectively blocking or passing angular sub-regions of scatter that arecollected by a collection optics subassembly 390.

FIG. 72 is a block diagram showing some of the relay lens assemblies 490contemplated by the present invention. As seen in FIG. 72, the types ofrelay assemblies 490, using glass cubes 462 to pass the beam of lightinto the detection units, may be used at each collector-detectorassembly 200 include: 1) unpolarized relay assembly 483, using anunpolarizing cube 452, 2) a fixed polarizing relay assembly 454 that isoriented in a fixed polarization state, and 3) a variable polarizationdevice 470, such as a rotational PBS analyzer 461. The fixed polarizingrelay assembly 454 and variable polarization device 470, collectivelyknown as polarizing relay lens assembly 450, may use either a polarizingbeamsplitter, such as cube 472 or a polarizing non-beamsplitter cubesuch as cube 456. The variable polarization device 470 in turn maycomprise, for example, a carousel cube assembly, such as rotatingcarousel polarization analyzer 451, for selecting between “P”, “S”, and“unpolarized” detector polarization states.

When inspecting surfaces bearing a film, however, such as semiconductorwafers with applied films, the three fixed detector polarization statesprovided by the rotating carousel polarization analyzer 451 may beinsufficient, because some films require ±45° as well as otherintermediate polarizer orientation angles in order to achieve the bestSNR. One approach to address this is to adjust the polarization of theincident beam in coordination with the detector polarization angle toachieve the optimal detector performance. The optimal SNR is related toboth the particle signal peak amplitude and the background levelobtained from the film surface. These parameters change for each type offilm that is present on the wafer surface. In accordance with anotheraspect of the invention, new rotational analyzer assemblies are providedto permit the intermediate polarizer orientation angles that arenecessary to achieve the optimal SNR when the surface producescircularly polarized scatter. The. rotational cube analyzer 461 androtational waveplate, fixed PBS analyzer 471.both described in detailbelow, both provide increased variation in polarizer orientation angles.

FIG. 27 shows a cut-away of a collector detector module 200 according tothe presently preferred embodiment described herein above, and whichcomprises a dual detector rotational polarizing cube analyzer 461 thatprovides P polarization, S-polarization and no polarization, as well asthe opportunity to provide combinations therebetween. The analyzer 461,referred to in FIG. 72, has a single polarizing beamsplitter 472 anddual detectors 497, 499. The collector detector assembly 200 on theright hand side is a back collector detector module, such as backcollector detector module 240A, and the one on the left is a wingcollector detector module, such as wing collector detector module 210A.The assembly in the center is the center collector detector module 220.The front collector detector module 230 also is shown. The PBS cube 472in each dual PMT assembly 461 can rotate around the detector collectoroptical axis.

Dual detector rotational polarizer analyzer 461 comprises a singlepolarization beam-splitter cube 472 and two PMT photodetectors 497, 499.The cube 472 can be rotated to the desired rotational angle around thedetector optical axis by manual means, not shown, or motorized means476. Therefore the PMT signals are directly associated with theorthogonal polarization states. If the polarizer is oriented so that PMT#1 sees “P” light, then PMT #2 will detect “S” light. If PMT #1 detects“+45°” light, then PMT #2 will detect “−45°” light. Furthermore, byelectrically adding the signals from both the PMTs 495, the resultingsignal is effectively the same as that obtained with no polarizerpresent (assuming the polarizer is lossless). Consequently, the assembly470 can simultaneously detect “P”, “S”, and “Unpolarized” light, or“+45°”, “−45°”, and “Unpolarized” light, or, more generally, “θ”,“θ−90°”, and “Unpolarized” light during a single scan of the wafer orsurface. The “unpolarized” signal is useful, for example, for scanningbare silicon surfaces and for some film inspection applications.

By adding the signals from the PMTs 495 for the unpolarized signal, onecan eliminate the need to mechanically exchange the polarized cube 472with an unpolarized cube 452. The equivalent optical path should bemaintained in the relay lens assembly 470 by including the unpolarizedcube 452. If the polarized cube 472 were removed and not replaced withan unpolarized cube 452, the spot would size would not be imagedcorrectly onto the PMT 495. The sides of the cube 472 or 452 are paintedblack as well, and it therefore acts as a baffle structure to furtherreduce stray light. By eliminating the need to exchange the cube 472 or452, the mechanical design can be simplified and this facilitatesmodularization.

Detection of COPs Using Polarization Information

The incorporation of an optical collection and detection subsystem 7comprising a series of collection and detection modules 200 into thesurface inspection system 10 enables more optimal use of the beamscanning subsystem 8. For example, some defects (such as scratches) aremore readily detectable in signals from a channel 600 formed from outputassociated with a wing collector 310A, 310B, when it is operated using“S” polarization, than, signals from a channel 600 formed from outputassociated with the wing collector 310A, 310B, when it is operated using“P” polarization, while particles are more readily detected in signalsfrom a channel 600 formed from output associated with the wingcollectors 310A, 310B, when they are operated using “P” polarizationthan signals from a channel 600 formed from output associated with thewing collector 310A, 310B, when it is operated using “S” polarization.By simultaneously providing both signals, the overall defect detectionperformance of the inspection system 10 can be improved.

When scanning bare polished wafers, the dual detector rotationalpolarization analyzer 461 preferably is oriented so that one PMT 495 is“P” and the other is “S.” In some applications, a variable polarizingassembly 470 is not necessary. In others, however, for example, such assome film inspection applications, polarizer orientation can be and ischanged routinely.

In summary, the collection and detection assembly 200 comprises acollector-detector field replaceable unit (“DFRU”) 811 configuration ofthe preferred embodiment of the present invention that is particularlyuseful in inspecting polished bare wafers using a fixed “P” and “S”relay assemblies 454 in each wing detector module 410A, 410B andunpolarized relay assemblies 483 (comprising unpolarized glass cubes452) in all of the other detector modules 420, 430, 440A, 440B. The DFRU811 configuration of the preferred embodiment of the current inventionthat is particularly useful in inspecting wafers on which films aredeposited uses variable polarizing relay assemblies 470 such asmotorized dual PMT rotational polarization analyzers 461, in the backdetector modules 440A, 440B and wing detector modules 410A, 410B, andunpolarized relay assemblies 483 (comprising unpolarized glass cubes452) in the center detector module 420 and front detector module 430.

A variable polarization analyzer 470 according to a further embodimentis shown in FIG. 28. In this analyzer design, the analyzer 470 comprisesa rotational waveplate, fixed beamsplitter polarization analyzer 471having a polarization beamsplitter (“PBS”) cube 472C and dual detectors495 that are rotationally fixed. A rotatable quarter waveplate (“QWP”)486 and half waveplate (“HWP”) 488 are located in front of the PBS cube472C. This enables the suppression of the background light, as describedin U.S. Pat. No. 6,034,776, which is herein incorporated by reference.By using a QWP/HWP combination, linear as well as elliptical polarizedlight can be substantially attenuated in one of the detectors 497, 499.By making the QWP/HWP combination rotatable, both linear and ellipticalpolarized light of selectable polarization mixes can be presented to thedetectors 497, 499. As before, an unpolarized detector signal can begenerated by adding signals from the two PMTs 495.

The polarization filters 450, 470 and non-polarizing assemblies 483 asdescribed here can be used in connection with any of the collectors usedin system 10, or any combination of them.

Front Collectors

As noted above, the optical collection and detection subsystem 7comprises means 250 for developing a light channel, for collecting thespecular beam reflected from the surface of the workpiece into a lightchannel 650, and means 260 for developing a dark channel, for collectingthe scatter from the workpiece surface S into a dark channel 655. Themeans 260 for developing a dark channel further comprises a series ofcollection and detection modules 200, one of which comprises a frontcollection and detection module 230. The front collection and detectionmodule 230 and the means 250 for developing a light channel are bothgenerally positioned in the path of the reflected incident beam.

Front collection and detection module 230 comprises a collector anddetector assembly having a front collector assembly 330 and a frontdetector assembly 430. Front collection and detection module 230 issimilar to the back collector and detector assembly 240A shown in FIGS.23-26. Objective lens optics 392 in the front collector 330 focus theincoming scattered (not specular) light to a slit 396, which operates asa field stop to absorb scatter outside of the illuminated localizedregion of the wafer being scanned. Light then passes to a relay lensassembly 490 in the front detector assembly 430. The front collector 330is similar to the back collector 430 shown in FIG. 25, with theexception that the slit 396 in the front collector 330 is disposed atthe appropriate Schiempflug angle, to match the angle of the image ofthe wafer surface W. FIG. 25, which shows the back collector anddetector module 240A, shows the slit 396 also arranged at theSchiempflug angle that corresponds to the angle of the back collector340A with respect to the wafer normal. The Schiempflug angle will bedifferent for the back, front, and wing collectors since they arepositioned at different angles with respect to the wafer normal. Thecenter collector 330 (or central collector 330) does not have aSchiempflug angle, because it is disposed normal to the wafer surface,and therefore has no Schiempflug condition. For more information aboutthe Schiempflug condition and how the Schiempflug angle is calculated,refer to FIG. 2.21 in Modern Optical Engineering, 2^(nd) ed., Warren J.Smith (McGraw-Hill, 1990).

Specular Beam Guiding System

In addition, the objective lens optics 392 in the front collector 330differs from the objective lens optics 392 in the back collector 340A inthat front collector objective lens optics 292 also has a light channelassembly 253 comprising an aperture (or hole) 251 (FIG. 22), which ispositioned in the front collector objective lens optics 292 at theintersection of the light channel axis LC to permit the specular beam topass through the optics 292.

The means 250 for developing a light channel also comprises a lightchannel assembly 253 that is positioned adjacent to the front. collector330 to receive the specular beam. As shown in FIG. 29, the light channelassembly 253 also comprises an input aperture 251 for receiving thespecular beam. The beam passes through an absorptive attenuation filter252 (composed of glass such as Schott NG4 from Schott Glass). Afterpassing through the attenuator 252, it passes through a 50/50beamsplitter 254, which splits or evenly divides the beam intotransmitted and reflected components. The transmitted component passesthrough a cylindrical lens 255, such as the SCX-50.8-127.1-C lens fromCVI Corporation (Albuquerque, N. Mex.), and is then received at a LinearPosition Sensitive Detector (LPSD) 256, such as the SL15 detector fromUDT Sensors, Inc. The LPSD 256 detects the centroid of the target spotTS. The cylindrical lens 255 ensures that the beam does not move duringthe AOD scan at the LPSD 256, which is located at the telecentric plane.

The reflected portion of the beam from the 50/50 beamsplitter 254 passesthrough a spherical lens 257, such as the Melles Griot 01ILPX282 planoconvex lens, and is then is received at a position sensitive detector258, such as the SPOT-9DMI segmented photodiode detector (or “quadcell”) from UDT Sensors, Inc. The quad cell detector 258 is sensitive tomovement of the reflected spot caused by both radial and tangentialtilt, which is useful for detecting slurry rings, slip lines, and otherpotentially non-scattering defects that exhibit low spatial frequencies.

Both differences in wafer height and wafer tangential tilt cause thespot to move on the LPSD 256. By linearly combining signals from thequad cell detector 258 and the LPSD 258, the signal component related totangential tilt can be removed from the LPSD signal, leaving only thesignal component related to workpiece height. Determining wafer heightrelative to the collection optics is important for properly computingthe x,y coordinates of wafer defects, since their apparent position withrespect to the beam changes with wafer height. Determining wafer heightrelative to the collection optics is also important to increaseknowledge of the wafer and the processes in which the wafer is involved.

Back Collectors

The optical collection and detection subsystem 7 according to anotheraspect of the invention comprises one or more wing collection anddetection modules positioned to collect at least one portion of thescattered light. It is preferable in some applications, such in particledetection, that there be two wing collection and detection modules 240A,240B, having, respectively, a wing collector assembly 340A, 340B and itsassociated wing detector assembly 440A, 440B. In some applications,however, it is desirable to collect signal from only one such backcollector, or more than two.

As with the center collector 320 and front collector 330, the objectivelens optics 392 in back collectors 340A, 340B focus the incoming photonsto slits 396, each slit 396, as the slit 396 in the center and frontcollectors, operating as a field stop to absorb scatter outside theilluminated region of the wafer. Light then passes to the relay lensassembly 490 in the back detector assembly 440A, 440B, associatedtherewith. The slit 396 in the back collector 340A, 340B is disposed atthe Schiempflug angle corresponding to the angle of the back collector340A, 340B with respect to the wafer normal.

The back collector module or modules are disposed in the backquartersphere BQ, outside the incident plane PI, and at or substantiallyat a maximum in the signal-to-noise ratio of defect scatter to surfaceroughness scatter. The wing collectors 310A, 310B may be positioned ator near a null or a minimum in to provide a reduction of noise fromRayleigh scatter. The reduction of Rayleigh scatter is discussed indetail below.

Center Collector

Surface inspection system 10 further also includes a center collectionand detection module 220 that, in this embodiment, comprises a centercollector 320 located directly above the desired spot on the workpiecesurface S (i.e., the center of the inspection table) whose optical axisis aligned to the vector that is normal to the surface S. The centercollector 320 in this embodiment is part of a collection and detectionsubsystem 7 as shown in FIGS. 23-26.

Center collection and detection module 220 comprises a collector anddetector assembly 200 having a center collector assembly 320 and a frontdetector assembly 420. As with the front collector 330, objective lensoptics 392 in the center collector 320 focus the incoming photon flux toa slit 396, which, as the slit 396 in the front collector, operates as afield stop to absorb scatter outside the region on the wafer that isilluminated by the scanned laser beam. Light then passes to a relay lensassembly 490 in the center detector assembly 420. The center collector320 is similar to the back collector 340A shown in FIG. 21, with theexception that the slit in the center collector 320 is disposed normalto the light passing through it because the wafer is disposed normal tothe light passing through the center collector 320; therefore there isno Schiempflug condition.

Wing Collectors

The optical collection and detection subsystem 7 according to anotheraspect of the invention comprises one or more wing collection anddetection modules positioned to collect a portion of the scatteredlight. It is preferable in some applications, such as those involvinginspection of bare or unpatterned semiconductor wafers, that there betwo wing collection and detection modules 210A, 210B, having,respectively, a wing collector assembly 310A, 310B and its associatedwing detector assembly 410A, 410B. In some applications, however,including but not limited to bare or unpatterned wafers, it is desirableto collect signal from only one such wing collector, or more than two.

As with the center collector 320 and front collector 330, the objectivelens optics 392 in wing collectors 310A, 310B focus the incoming photonsto slits 396 in wing collectors 310A, 310B, each slit 396, as the slit396 in the center and front collectors, operating as a field stop toabsorb scatter outside the illuminated region of the wafer. Light thenpasses to the relay lens assembly 490 in the wing detector assembly410A, 410B, associated therewith. A wing collector 310A, 310B is similarto the back collector 340A shown in FIG. 21, with the exception that theslit 396 in the wing collector 310A, 310B is disposed at the Schiempflugangle corresponding to the angle of the wing collector 310A, 310B withrespect to the wafer normal rather than at the Schiempflug anglecorresponding to the angle of the back collector 340 with respect to thewafer normal.

The wing collector module or modules are disposed in the frontquartersphere FQ, outside the incident plane PI, and at or substantiallyat a maximum in the signal-to-noise ratio of defect scatter to surfaceroughness scatter. The wing collectors 310A, 310B may be positioned ator near a null or a minimum in surface roughness scatter relative todefect scatter for scattered light from the surface S, or the Pcomponent thereof. For example, wing collectors 310A, 310B may bepositioned at about a minimum in the bidirectional reflectancedistribution function (“BRDF”) for the surface when the incident beam isP polarized and the detector assembly 400 is also P-polarized. Thecalculation of the BRDF is discussed in detail below.

It is desirable to locate the wing collectors 310A, 310B at suchlocations, for example, because, at these locations, the haze, which maybe defined to be the diminished atmospheric visibility that results, inthe case of a surface inspection tool, from light scattered from asurface, and which determines background noise (due to BRDF) isminimized, but the defect scatter signals remain, preferably at or neara maximum relative to the noise. The haze or background noise (due toBRDF) is minimized because, when, as in the present invention, thecollection optics contains a polarizer that is oriented in local “P”polarization, the light scattered from the surface has “S” orientation.The polarizer that is oriented in local “P” polarization thuscounteracts the haze or background noise that has an “S” orientation.

Thus, collection at or near a null or a minimum in surface roughnessscatter relative to defect scatter, for example, from a defectperspective, at a maximum in the signal to noise ratio of defect scatterto surface roughness scatter when the incident beam is P polarized, or,from a surface roughness scatter perspective, when the surface roughnessis at a relative minimum for scattered light from the surface Sresulting from the bidirectional reflectance distribution function(“BRDF”) of the surface S, or the P component thereof, when the incidentbeam is P polarized and the detector assembly 400 is also P-polarized,provides an enhanced signal to noise ratio for these signals. This isillustrated by FIGS. 30 and 31. These figures show a BRDF forP-polarized light incident on the workpiece surface S at 65° withrespect to the normal vector N, and where beamsplitter 472 is configuredto pass P-polarized scattered light to the detector while blockingS-polarized light. FIG. 30 shows the BRDF using linear intensity, andFIG. 31 uses log intensity. In both, the y-axis is representative of thespherical coordinate theta, or an angle of elevation, and the x-axis isrepresentative of spherical coordinate phi, or an azimuthal angle. Thelocation (0,0) is normal to the wafer, and pointing along the opticalaxis of the center collector 320. From these graphs one may identify thelocal minima or nulls of the BRDF (hereinafter referred to asBRDF_(MIN)), and correspondingly select a location for the wingcollectors, in terms of azimuth and elevation. Referring particularly toFIG. 31, one can see BRDF_(MIN) as the darker region extending upwardfrom the x-axis to the point (0,80) and then downward toward the x-axisagain. Points 910A, 910B identify one combination of angles of elevationand azimuth for placing, respectively, the collectors 310A, 310B atlocations in which haze which determines background noise is minimized.Specifically, the coordinates of points 910A, 910B define the angles ofelevation and azimuth for such preferred placement. Once the decision ismade to place the wing collectors 310A, 310B at a selected angle ofelevation 912, the collectors' azimuthal placement is determined by thex-coordinate of the two locations of the BRDF_(MIN) associated with theelevation angle 912 s, namely azimuthal angles 914, 916, and thecombination of desired angles are defined by the coordinates of points910A, 910B.

The optical collection and detection subsystem 7 further comprises apair of wing collection and detection assemblies 210A, 210B positionedin the front quartersphere FQ but outside the incident plane PI. Wingcollectors 310A, 310B are substantially identical to one another. Eachcomprises a portion of a collection and detection assembly 200 as shownin FIGS. 23-26. Wing collectors 310A, 310B in this embodiment arelocated symmetrically with respect to the incident plane PI, and whenthey have identical focal lengths, they are equidistant from a point onthe light channel axis LC and equidistant from the surface S of theworkpiece W. This also applies where multiple pairs of wing collectorsare used. Wing collectors 310A, 310B are positioned to receive a desiredand preferably optimal or near optimal amount of light scattered fromdefects on the workpiece surface S. By positioning the wing collectionand detection assemblies 210A, 210B out of the plane of incidence PI,the amount of light coupled into the wing detector assemblies 410A, 410Bassociated with wing collectors 310A, 310B due to Rayleigh air scatteris reduced, thereby reducing the background light and improving thesignal to noise ratio (SNR).

The optical collection and detection subsystem 7 uses P-polarizedincident light at 65° of incidence, as noted above. The scattered lightfrom an optically smooth surface exhibits a minimum at a specific angleif the optical detector detects only P-polarized light since the surfaceroughness scatter from the wafer is S-polarized when the incident beamis P-polarized for the desired wing collector locations. This is onlytrue for surfaces that exhibit Rayleigh-Rice scatter, as described inOptical Scattering, Measurement and Analysis, 2nd ed., John C. Stover,(SPIE Optical Engineering Press 1995)(hereinafter the Stover reference).This effect is shown in the plots in FIGS. 24 and 25. These plots werederived from Equations 4.1 and 5.12-5.17 in Optical Scatter. The null isthe multi-dimensional equivalent of the Brewster angle. The location ofthe null, therefore, is dependent upon the index of refraction of thesurface.

The wing collectors 310A, 310B of the wing collection and detectionmodules 210A, 210Bs of the optical collection and detection system 7 arealso designed and placed to provide, along with the front collector 330of the front collection and detection module 230, symmetrical and nearlycomplete collection of forward scattered light. This can improve thescratch detection performance of the system.

The collection angle of wing collectors 310A, 310B in the presentembodiment are about 26° (half angles of about 13°). As stated above,the spherical angle corresponding to the desired surface roughnessscatter null will be dependent on the index of refraction of thematerial. For some types of surfaces, it may be desirable to increasethe size of the wing collectors 310A, 310B to 30 degrees or more andadjust the angular position of the optical axis for optimal SNR. Notethat the detector assembly 400 design incorporates selective subaperturemasking (“virtual mask 131,” described below), which can enableselective subaperture collection to collect light only from angularregions where the SNR is highest.

The first wing collector 310A is positioned with an azimuth angle withrespect to the light channel axis LC of about 5 to 90°, and the secondwing collector 310B is positioned at an azimuth angle with respect tothe light channel axis LC of about −5° to −90°. The azimuth angles usedin the presently preferred embodiments are about +50 and −50 degrees. Itshould be noted that, in referencing the positioning of the collectors300 herein, the angular position of the collector 300 is measured to acentral point on the central axis of the collector 300, i.e., an opticalaxis of the lens corresponding to the axis of the optical path of thebeam as it passes through the center of the lenses in the collector'sobjective lens optics 392.

The wing collectors 310A, 310B preferably have an elevation angle withrespect to the surface S of the workpiece W of about 30° to 90°. In thepresently preferred embodiments and method implementations, theelevation angle of wing collectors 310A, 310B is about 45°.

In the preferred embodiments and implementations, a polarizingbeamsplitter in each of the wing collection and detection modules 210,210B, such as the beamsplitter 472 illustrated in FIGS. 23-26, isdisposed in the relay lens assembly 490 at the input to each wingdetector assembly 410A, 410B that is associated with a wing collector310A, 310B, i.e., in the optical path of the region between the desiredspot and the wing detector assembly associated with a wing collector orcollectors 310A, 310B. This enables one of the detectors 497, 499 of thecollection and detection assemblies 210A, 210B to receive solelyP-polarized radiation, and thereby take full advantage of this effect.

In accordance with this aspect of the invention, the method for locatingthe positions of the wing collectors 310A, 310B can be further explainedand elaborated upon. U.S. Pat. No. 6,034,766 describes the use of aplurality of small solid-angle collectors over the surface of ascattering hemisphere to detect defects on a microrough surface. Thepatent indicates that large number of these collectors should beemployed to cover a large solid angle. The patent also suggests that apolarization analyzer should be employed at each collector to beorthogonal to the scatter from microroughness so as to maximizesignal-to-noise ratio.

The '776 patent fails to take into account two concerns that often arepresent in such systems. First, while each collector can be set to be“microroughness-blind,” the detectors will still be subject to Rayleighscatter from the molecules of air near the surface of the workpiece.Rayleigh scatter is the scatter of light off the gas molecules of theatmosphere, principally Nitrogen for normal air. When surface inspectionsystems are operated in air, the illumination source generates Rayleighscatter. This effect can be reduced by operating in partial vacuum, orby use of a gas with lower scattering cross-section such as Helium.Because both of methods for reducing Rayleigh scatter are difficult andexpensive to implement, typically surface inspection systems areoperated in air. Therefore, each collector in such systems has aconstant background flux caused by Rayleigh scatter from the atmosphere.Even though Rayleigh scatter is a relatively small scatter componentcompared to surface roughness scatter, it is more significant,especially in the back collector of a multi-collector surface inspectionsystem such as system 10, when the surface scatter level is relativelylow, for example when wafer surfaces with an extremely good polish areinspected. (See “A Goniometric Optical Scatter Instrument forBidirectional Reflectance Distribution Function Measurements withOut-of-Plane and Polarimetry Capabilities”, Germer and Asmail, from“Scattering and Surface Roughness,” Z.-H. Gu and A. A. Maradudin,Editors, Proc. SPIE 3131, 220-231 (1997)). Second, building a systemwith a large number of collectors usually is expensive, difficult to setup and difficult to maintain. Furthermore, by setting the polarizer ineach detector to minimize the background from surface roughness scatter,some detectors will also substantially reject important defect signal aswell. The shot noise of the low level signals result in large defectvoltage variations that could be confused with voltage signalsrepresentative of defects.

Because of the high cost associated with using a large number ofcollectors, it is desirable to reduce the number of collectors in thesystem. In accordance with this aspect of the invention, this objectivecan be achieved by placing the collectors at the locations on thescattering hemisphere where they can achieve the greatest advantage.Thus, the collectors are placed where they will have the highestsignal-to-noise ratio for a selected range of workpiece surfaces andmaterials.

Because of the presence of the Rayleigh scatter from molecules in theatmosphere, each collector will have a constant background flux due tothe Rayleigh scatter. Measuring photon flux has an inherent unavoidablenoise associated with it called “shot noise.” It is expected that shotnoise will be present in any surface inspection system. When thecollectors are operated in air, the shot noise in the output associatedwith P-polarized wing collectors tends to be dominated by Rayleighscatter. The shot noise in the output associated with the backcollectors tends to be dominated by surface roughness scatter.

Shot noise consists of random fluctuations of the electric circuit in aphbtodetector, which are caused by random fluctuations that occur in thedetector or by fluctuations in the number of electrons (per second)arriving at the detector. The amplitude of shot noise increases as theaverage current flowing through the detector increases. The fluxmeasurement is really counting a rate of how many photons per second arecollected by the detector. The longer the period of counting, the moreaccurately one can measure the rate. It can be shown that thepower-equivalent noise from the Rayleigh scatter is given by:σ_(Rayleigh) ²=2E _(photon) P _(Rayleigh) χBW/QE,where

-   -   σ_(Rayleigh) ² is the variance of the measured Rayleigh scatter        at the detector (in Watts²),    -   E_(photon) is the energy of each photon in Joules,    -   P_(Rayleigh) is the Rayleigh scatter present at the detector (in        Watts),    -   QE is the quantum efficiency of the detector (dimensionless),    -   BW is the bandwidth of the measurement system (in Hz—equivalent        to 1/sec), and    -   χ is the excess noise factor of the detector (dimensionless).        Furthermore, this noise is nearly Gaussian whenever

$\frac{{QE}\;\sigma_{i}^{2}}{E_{photon}}\operatorname{>>}1.$For practical scattering systems, this ratio is typically severalhundred. We see that the RMS noise level (square root of variance) canbe given by:

${\sigma_{Rayleigh} = {K\sqrt{P_{Rayleigh}}}},$where K encompasses system contributions to noise, such as bandwidth,quantum efficiency, excess noise factor and photon energy. A defectparticle will scatter with power P_(particle). To maximize thesignal-to-noise ratio (SNR), we maximize

$\frac{P_{particle}}{K\sqrt{P_{Rayleigh}}}.$Note that the optical powers P_(particle) and P_(Rayleigh) are functionsof incident wavelength, incident polarization, particle size, particlematerial, substrate material, incident declination angle, collectorsolid angle, collector declination angle and collector azimuth angle.These scatter powers are also controlled by the polarizer at thecollector, which preferably is set to null the scatter frommicroroughness. For typical designs, the incident wavelength andincident declination angle are fixed. The collector solid angles arealso fixed, and typically small. We now want to find the locations toplace collectors 300 on the scattering hemisphere that maximize SNR.

In this illustrative example, silicon is used as the substrate orsurface to be inspected. Using a beam having a wavelength fixed at 532nm, an incident declination of 65 degrees, and p-polarization for theincident beam, the SNR plots for a small variety of particle sizes andmaterials can be shown. While the actual SNR values depend heavily onparticle size and particle material, the scattering hemisphere locationsof maximum SNR change very little. This can be used in accordance withthis aspect of the invention to set the locations of the wing collectors310A, 310B, e.g., using “microroughness-blind” collectors that arepositioned according to their maximum SNR regions based solely uponincident wavelength, incident declination, incident polarization andsubstrate material.

Using this method, and in the particular case of a 532 nm beam source, a65 degree incident declination, and p-incident polarization on a siliconsubstrate, the wing collectors 310A, 310B are placed in the regions of40-70 degrees of declination, and either 40-70 degrees azimuth or290-320 degrees azimuth to maximize SNR.

In order to fit the collectors 300 into the space above the wafer W, itmay become necessary to cut sections out of the collectors 300. Cuts maybe seen in FIG. 22. As noted above, the output signal associated withthe wing collectors 310A, 310B may be combined with output signalsassociated with selected other collectors 300 in order to provideimproved defect detection and/or classification. Specifically, as willbe described in further detail below, the back collectors 340A, 340B andP-polarized wing collectors 310A, 310B receive proportionately moresignal from particles on the wafer surface than they receive from pitson the wafer when scanning defects <100 nm in size and using P-incidentpolarized light. Therefore, in order to facilitate the identification ofpits on the surface of the wafer, it is preferable to cut as little aspossible from the center collectors 320 (preferably no more than about10%).

Collector system with Back and Wing Collectors

In accordance with another aspect of the invention, an opticalcollection system is provided for use in a surface inspection system 10such as those described herein. In this aspect of the invention as inothers, the surface inspection system 10 has an incident beam projectedthrough a back quartersphere BQ and toward a spot on the surface S ofthe workpiece W so that a specular portion of the incident beam isreflected along a light channel axis LC in a front quartersphere FQ. Theoptical collection system according to this aspect of the inventionpreferably comprises a subsystem of a surface inspection system 10.Thus, to illustrate and further describe this aspect of the invention, apresently preferred optical collection system embodiment will bedescribed in the form 5 of an optical collection subsystem 380 of system10. It will be appreciated, however, that the optical collection systemis not necessarily limited in this respect.

In accordance with this aspect of the invention, an optical collectionsubsystem is provided that comprises a plurality of back collectorspositioned in the back quartersphere BQ and outside the incident planePI for collecting scatter from the workpiece surface. The number of backcollectors may vary depending upon the application. Preferably there aretwo such collectors, such as collectors 340A, 340B. The back collectors340A, 340B preferably are substantially identical to one another. Wheremore than two back collectors 340A, 340B are employed, it is preferredthat they be used in pairs, and positioned symmetrically with respect toone another and with respect to the incident plane for a given pair. Theback collectors 340A, 340B also preferably are located symmetricallywith respect to the incident plane PI, and, when they have identicalfocal lengths, they are equidistant from the incident plane PI and thedesired spot on the workpiece surface S, in general or at least forgiven pairs of the collectors 340A, 340B.

As with the center collector 320 and front collector 330, objective lensoptics 392 in the back collectors focuses the incoming beam to a slit396, which operates as a field stop to absorb scatter outside theilluminated scan region of the wafer being scanned. Light then passes toa relay lens assembly 490 in the detector assembly 400. While the slit396 in the center collector 320 was disposed normal to the light passingthrough it, in the back collector, the slit 396 is arranged at theSchiempflug angle, to accommodate for the angle of the back collectorwith respect to the wafer normal. FIG. 25, which shows the backcollector 340A, shows the slit 396 arranged at the Schiempflug anglecorresponding to the angle of the back collector 340A with respect tothe wafer normal.

Back collectors 340A, 340B according to this aspect of the inventionpreferably are positioned at azimuth angles of up to about 90°, and morepreferably about 10° to 90°, with respect to incident plane in the backquartersphere BQ. These angles equate to about 90° to 180° and about 90°to 170°, respectively, with respect to the light channel axis LC.Azimuth angles of at least about 45° to 55° are even more preferred,particularly in semiconductor wafer surface inspection systems, forexample, such as system 10.

The presently preferred elevation angles for back collectors 340A, 340Baccording to this aspect of the invention are about 35-60 degrees withrespect to the workpiece surface S. More preferably, the elevationangles of the back collectors 340A, 340B are about 53° with respect tothe workpiece surface normal vector.

The collection angle of back collectors 340A, 340B according to thisaspect of the invention preferably are about 20° to about 60°(i.e., halfangles of about 10° to about 30°), and more preferably about 60° (i.e.,half angle of about 30°).

As implemented in the presently preferred embodiments, an opticalcollection subsystem is provided with two back collectors 340A, 340B.Collector 340A is positioned at an azimuth angle of 55° with respect tothe projection of the incident beam on the surface in the backquartersphere. Collector 340B is positioned at an azimuth angle of about−55° with respect to this same incident beam projection. In thisembodiment of an optical collection subsystem, collectors 340A, 340B areequidistant from the surface S of the workpiece W, and each has anelevation angle with respect to the desired spot on the surface S ofabout 53°. When scanning polished wafer surfaces, the relay lensassembly 490 in back detectors 440A, 440B associated with backcollectors 340A, 340B uses unpolarized cubes. When scanning films, arotating analyzer 472 such as described above can be used to minimizebackground from the film surface in some applications.

Front collectors, Contd.

Switchable Edge Exclusion Mask

In accordance with another aspect of the invention, a switchable edgeexclusion mask 132 is provided in the front collector 330 in order tocover the region of the objective lens optics 292 between the specularbeam hole 251 and the outer edge of the lens L1. It is frequentlydesirable to scan the edge of a workpiece W, e.g., a silicon wafer, tocalculate or determine such things as the placement of the wafer withrespect to the center of rotation, to look for edge chips, etc.Unfortunately, the wafer or workpiece edge typically is beveled in sucha way that the laser beam may be reflected directly into the frontcollector 330. This can cause the detector or detectors 497, 499 in thedetector 430 associated with the collector 330 to saturate, and canactually damage them in some cases, e.g., the anode of the PMTs 495. Tolimit or prevent this, an edge exclusion mask 132 according to thisaspect of the invention may be placed in front of the front collector330 to absorb the specularly reflected beam as it scans across the edgeof the wafer W.

In accordance with this aspect of the invention, switchable edgeexclusion mask 132 is provided. In an illustrative but not necessarilypreferred embodiment of this aspect of the invention, switching isperformed electro-mechanically.

Using an edge exclusion mask 132 can substantially reduce the edgeexclusion zone on the wafer (the region over which data can be reliablycollected near the edge of the wafer). Unfortunately, however, it alsocan reduce the sensitivity of the front collector 330 to scratches thatare perpendicular to the AOD scan direction. An example of this is shownin FIG. 32, which shows a scratch distribution plot. This problem can beminimized using a switchable edge exclusion mask 132 as describedherein.

A mask 132 according to this aspect of the invention is illustrated inFIG. 33. Mask 132 is normally energized so that it is outside thecollection field of the front collector 330 when scanning the interiorof the workpiece surface S. This maximizes the front collectorcollection efficiency, and enables complete collection of scratches thatare perpendicular to the AOD scan direction. When the AOD scanapproaches within 1-3 mm of the edge of the scan, the mask 132 iselectromechanically moved in front of the lens and blocks the scatterand reflection from the wafer bevel.

The mask 132 is designed to cover the region of the front collector lensbetween the specular beam hole and the outer edge of the lens. The mask132 is connected to an electromechanical means 133 having an edgeexclusion actuator 137 for moving the edge exclusion mask 132. A sensor136 is employed to sense the position of the mask, enabling the controlcomputer 500 (described more fully herein below) to sense if the edgeexclusion actuator 137 is working correctly. The electromechanical means133 for moving the edge exclusion mask 132 could comprise a rotarymotor, a two-position motor, a stepper motor, a DC servo motor, or apneumatic means. In an illustrative but not necessarily preferredembodiment of this aspect of the invention, as illustrated in FIG. 33,the electromechanical means 133 comprises a drive mechanism 134 with adrive mechanism stage 138 and an air drive 135 for holding and movingthe edge exclusion mask 132.

Edge exclusion masks 132 in accordance with this aspect of the inventionadvantageously can enable one to obtain a small edge exclusion zone nearthe edge of the wafer or workpiece surface S without sacrificing overallfront collector sensitivity.

Moveable, Switchable Virtual Mask

In accordance with still another aspect of the invention, a moveable,switchable virtual mask 131 is provided. Most of the optical scatterfrom a semiconductor wafer or similar workpiece is produced by thelowest spatial frequencies, and is therefore confined to a small angularrange around the specular beam in the forward scatter direction. Thisbackground scatter tends to dominate the signal detected by the frontcollector PMT 495, masking the presence of critical defects, such asmicroscopic scratches, that comprise higher spatial frequencies. Defectsassociated with higher surface structure spatial frequency contentgenerally scattered light into larger angles with respect to thespecular beam. It is possible to partially or fully absorb or otherwiseexclude excessive scattered light associated with low surface structurespatial frequency from the wafer surface using an appropriate maskingdevice.

In past inspection systems, in order to improve the defect detectionperformance of the front collector, an elliptical mask was installed infront of the lens to block this scatter. The mask was ellipticallyshaped in order to block the light scattered from surface structures oflow spatial frequency relative to the collection geometry for thein-scan and cross-scan directions (based on the 2D grating equationexpressions in the Stover reference, page 75). However, the placement ofthe black anodized aluminum mask in front of the lens in the frontcollector tended to reflect scatter back to the wafer, thereforeintroducing additional scatter into other detectors 497, 499.

In addition, when introducing such a mask, however, it is oftenundesirable to position the mask in front of the collection lens sincethis would scatter light back to the test surface. The mask in thisinstance can block desired light that includes important informationabout the surface. For example, it is desirable to detect “flatparticles” shallow bumps or dimples whose aspect ratio is large, withdiameters greater than 1 micron and heights of a few nanometers. Thesedefects scatter light associated with a lower surface structure spatialfrequency range (near the specular beam) than do typical sphericaldefects <100 nm in diameter. Therefore, when scanning a wafer for flatparticles or dimples, it may be desirable to use a mask that blocksscattered light associated with higher or lower surface structurespatial frequencies than those associated with the defects of interestin the front collector but allows the scattered light associated withthese particular defects to pass through to the detector 497 or 499.

In order to enable the system to optimally detect either small particlesor flat particle defects, a moveable, switchable virtual mask 131 isprovided according to this aspect of the invention. Switching and movinga virtual mask 131 can allow the user to select the angular range oflight that must be masked based on the surface structure spatialfrequency content of the wafer or other like surface, and can allowoptimization of defect sensitivity for defects that have unusual angularscatter distributions.

A virtual mask 131 according to a presently preferred yet merelyillustrative embodiment of this aspect of the invention is shown inFIGS. 34 and 36. The virtual mask 131 comprises a black or otherwiselight absorbing glass mask, preferably elliptical in shape, which isoptically bonded to the glass cube 462 that is located in the detectorrelay lens assembly 490 of the detector module 430. Alternatively, themask 131 could comprise a black anodized aluminum sheet metal mask thatis positioned in a correct optical imaging position in the detectormodule 430, which is located along the beam path after the collectormodule 330.

The virtual mask 131 is used to block the scatter near the specularbeam. As shown in FIG. 34, scattered light from the wafer or otherworkpiece surface S is collected by the collector 200 over the solidangle subtended by the objective lens optics 392. After the lighttravels through the objective lens optics 392, it passes through a slit396, and then to the cube 462, which may comprise polarizingbeamsplitter 472, polarizing cube 456, or unpolarized cube 452. Lightthat reaches the virtual mask 131 is blocked from reaching the PMT 495.

Scatter that is reflected off of the mask 131 is minimized by the mannerin which the virtual mask 131 is attached to the cube 462 and by theplacement of the mask 131 after the slit 396. Since the black glasspiece that comprises the virtual mask 131 is bonded to the cube glass396 with index-matching optical cement, the optical interface betweenthe black glass mask 131 and cube 396 exhibits minimal scatter. Inaddition, any residual scattered light that bounces back from the mask131 must pass through the slit 396 to pass back through the collector330 and arrive at the wafer W, and is therefore substantially reducedrelative to prior known systems.

In the presently preferred yet merely illustrative embodiment, as shownin FIG. 36, the black glass piece mask 131 is located at an x-y positionwithin the front collector detector module 230 slightly off the opticalaxis, mapping the specular beam, in order to force the mask to becoincident on the specular beam hole 251. The location of the blackglass mask 131 at a z-position such that the real image of the blackglass mask 131 is located between the two aspheric objective lenses L1,L2 causes the glass piece to comprise a “virtual mask” rather than aphysical mask that is in front of the objective lens optics assembly 392of the front collector 330.

The virtual mask 131 shown in FIGS. 34 and 36 is fixed (bonded to theback of the cube). It may be rendered switchable by exchanging a mask(such as an aluminum mask) in/out of the desired position, for example,by manually replacing the glass cube 362 in the detector module 430. Theonly way to move it is to either or Alternatively, the virtual mask 131is rendered switchable and moveable by moving the glass cube 362 intoanother location, for example, using a carousel approach. As notedabove, in the presently preferred yet merely illustrative embodiment ofthis aspect of the invention, the virtual mask 131 is a black glasspiece, which is optically bonded to the glass cube 362 that is locatedin the relay lens assembly 490 of the detector module 430. As notedabove, in the presently preferred yet merely illustrative embodiment ofthis aspect of the invention, the polarizing beam splitter glass cube472 is switchable to provide variable polarization. As the glass cube472 is moved, so is the glass piece mask 131 attached to it.

Although the virtual mask 131 according to the presently preferredembodiment is depicted only in the front detector module 430, it can beused in any of the detector assemblies 400 to either limit the solidangle collection range of the detector module 400 or to mask offunwanted solid angles. The solid angle range of collection is specifiedby the size and shape of the virtual mask 131. Annular virtual masks canbe used to reduce the effective solid angle of collection of thedetector module 400. Other shapes may be used to collect the desiredlight from a sub-region of the lens assembly. The virtual mask 131 couldalso be employed in the wing collection and detection modules 210A, 210Bto limit collection of the scattered light associated with surfacestructure spatial frequency spectrum to only a defined sub-region of aworkpiece region wing. The wing collection and detection modules 210A,210B of preferred system 10 as described herein are optimized to detectlight within the BRDF null (BRDF_(MIN)) while achieving reasonablesensitivity to 45 degree scratch signals. As shown in the surfacestructure spatial frequency plot of FIGS. 37-38, the wing collectors aredesigned to cover a portion of the surface structure spatial frequencyspectrum between the front collector 330 and back collectors 340A, 340B.FIG. 37 shows the front collector surface structure spatial frequencyspectrum coverage 143, the wing collectors' surface structure spatialfrequency spectrum coverage 141A, 141B, and a portion of the centercollector surface structure spatial frequency spectrum coverage 142.FIG. 38 shows the back collectors' surface structure spatial frequencyspectrum coverage 144A, 144B and the remainder of the center collectorsurface structure spatial frequency spectrum coverage 142.

If in a particular application it is desirable or necessary to achievemore complete wing collectors' surface structure spatial frequencyspectrum coverage 141A, 141B in the region between the front collectorsurface structure spatial frequency spectrum coverage 143 and backcollectors' surface structure spatial frequency spectrum coverage 144A,144B, in order to provide fuller coverage of this region, an asphericlens design can be used in this region if the azimuth angle is increasedto about 60 degrees. In certain semiconductor wafer applications thereare primarily three reasons for collecting a sub-region of thisregion: 1) the BRDF null would be located to one side of the lens if theazimuth angle were set to 60 degrees, 2) the location of the BRDF nullis dependent upon the index of refraction of the material, therefore itcan move to slightly different locations for different materials, and 3)Rayleigh scatter adds an additional background contribution that canincrease the overall background light collected by the wing (or other)collector and can partially shift the effective location of the BRDFnull. It also may be desirable to use the virtual mask 131 in the relayto collect a sub-region of the wing collector surface structurefrequency spectrum 141A, 141B to compensate for these three effects. Aswith the virtual mask 131 in the front collection and detection module230, the virtual mask 131 could be moveable or it could be selectable ina carousel fashion in order to collect a sub-region of the wingcollector solid angle for optimal SNR.

Signal Processing Subsystem

Signal Processing Architecture

A number of the systems and their illustrative but not necessarilypreferred embodiments as disclosed herein comprise a processingsubsystem or module 19 operatively coupled to an optical collection anddetection subsystem or module 7 for processing the signals generated bylight detection. This processing module 19 performs processing on thesignals obtained from the optical collection and detection subsystem 7to provide desired information concerning the surface S of the workpieceW under inspection, such as its geometry, characteristics, defectinformation, and the like. The processing system 19 as implemented inthe illustrative but not necessarily preferred embodiment comprises acontroller such as system and processing unit 500.

As best illustrated in the perspective view of FIGS. 3 and 4, thesurface inspection system 10 preferably is computer controlled. Thesystem controller and processing unit 500 operates the inspection system10 under the supervision and direction of a human operator, stores andretrieves data generated by the system 10, and performs data analysispreferably responsive to predetermined commands. The relative positionof the article being inspected is communicated to the system controller500 via motors, not shown, and encoders, not shown, mounted thereto. Theposition data is transmitted to the gauge synchronization control 186,which responsively drives the AO deflector 100 via an AOD scan driver950.

As understood by those skilled in the art, data signals from thecollectors are conventionally electrically communicated to theprocessing electronics 750. The processing electronics 750 couldcomprise digital electronics (not shown) and analog electronicscomprising an Analog Combining Board (not shown) for processing thesignals, such as that described in the '701 patent. In a presentlypreferred yet merely illustrative embodiment, the signals are processeddigitally using the data processing system shown as system controllerand processing unit 500 in block diagram form in FIG. 46.

As shown in FIGS. 46 and 48, a data processing subsystem or module 19for use in inspecting a surface of a workpiece has a data acquisitionsystem 54 comprising a plurality of data acquisition nodes 570 (DANs570) connected by a communication network to a data reduction system 55comprising a plurality of data reduction nodes 670 (DRNs 670). A systemcontroller and processing unit 500 is connected to the data reductionsystem 55 via an interface or switch 660 arranged for a communicationnetwork or other system controller and processing unit 500communication. The system controller and processing unit 500 is operatedusing keyboard 16, mouse 18, etc., and it presents output on display 20or other suitable peripherals, e.g., a printer. The system controllerand processing unit 500 outputs the data representative of the selectedset of collectors to a channel analysis system 520 through System I/O530.

Channel Definition and Channel Combining

Output from the optical collection and detection subsystem 7 isorganized into defined channels 600. FIG. 47 lists a set of channels 600that could be formed for the presently preferred but merely illustrativeembodiment described herein. Certain channels 600 comprise the set ofdata comprising the output associated with an individual collectormodule 300, such as the center channel 620 formed from data from thecenter collection and detection module 220 and the front channel 630formed from data from the front collection and detection module 230.

Other channels 600 are formed from the set of data comprising the outputassociated with a combination of collection and detection modules 200,such as the spherical defect channel 615, which would be particularlysensitive to the detection of small spherical objects such as 50 nmpolystyrene latex spheres (PSLs) and defects with like geometries,formed from data from the wing collection and detection modules 210A,210B when operated in P-polarized format and the dual back collectionand detection modules 240A, 240B. In addition, channels 600 comprise theset of output data associated with selected combinations of collectormodules 300 operated in a selected format or in which the data areprocessed using a selected method. For example, back combined (CFT)channel 641 is formed from output data associated with back collectionand detection modules 240A, 240B when they are combined using a selectedsignal combining CFT method 812 involving first combining, thenfiltering/thresholding the data (the method is described in more detailbelow). Similarly, the wing combined (CFT) P channel 610P and wingcombined (CFT) S channel 61 OS are formed from data from the wingcollection and detection modules 210A, 210B when the resulting data areoperated in a selected polarization format (P or S, respectively) andthe resulting data combined first by combining, thenfiltering/thresholding. Generally, channels C1 through CN could beformed from the output data associated with any individual collectionand detection module 200 or any desired combination of collection anddetection modules 200.

Light channels 650 are similarly formed with output data collected fromthe light channel assembly 253, which has as input the specular beamreflected from the surface S of the workpiece W. Light channels 650comprise, specifically, the extinction channel 650EXT, the radialchannel 650R, the tangential channel 650T, and the height/reflectedpower channel 650H/RP.

As noted herein, an illustrative channel 600 could comprise thespherical defect channel 615 defined from the combination of wingmodules 210A, 210B when operated in P-polarized format and the dual backmodules 240A, 240B, which would be particularly sensitive to thedetection of small spherical objects such as 50 nm polystyrene latexspheres (PSLs) and defects with like geometries. Channels are defined tocomprise sets of collectors, using any of the combinations of collectorsets described herein (such as channel 615) or any other desiredcombination, and output signals associated with the sets of collectorsare combined according to any conventional methods or the methodsdescribed herein into output to be associated with the defined channel.The resultant output may be analyzed using any methods such as thosedescribed herein or any known defect detection method, such as thosedescribed in U.S. Ser. No. 10/864,962, entitled Method and System forClassifying Defects Occurring at a Surface of a Smooth Substrate UsingGraphical Representation of Multi-Collector Data, which is assigned toADE Corporation of Westwood, Mass. and which is herein incorporated byreference.

It should be noted that the present invention should not be limited tothe embodiment of the present invention, in which channels 600 areformed from combinations of collectors 200 disposed at selectedlocations in the space above a workpiece surface. It should be notedthat the present invention should not be limited to the collectors asdescribed above. For example, collectors 300 have collection opticssubassemblies 390 that direct the scatter to detectors 400.Alternatively mirrors could be used to direct the scatter to detectors400. In addition, the present invention should not be limited todefining channels from collector response to light scattered fromsurface structural conditions.

Fundamentally, the invention involves combining signal representative oflight of selected characteristics scattered from surface structuralconditions, with characteristics comprising, for example, selectedpolarization and/or presence in a selected solid angles over a workpiecesurface. As an example, the spherical defect channel 615 is preferablyformed from signal representative of P-polarized scatter collected at aplurality of solid angles over a workpiece surface in the frontquartersphere FQ and from signal representative of scatter collected atplurality of solid angles over a workpiece surface in the backquartersphere BQ of the space above a wafer, outside the incident planePI.

Preferably, the plurality of solid angles in the front quartersphere FQrepresent locations at or substantially at a maximum in thesignal-to-noise ratio of defect scatter to surface roughness scatter,or, from a surface roughness scatter perspective, when the surfaceroughness is at a relative minimum in a bi-directional reflectancedistribution function when the incident beam is P polarized. Morepreferably, the solid angles represent two locations, preferablysubstantially identical to one another and positioned symmetrically withrespect to one another and with respect to the incident plane. In system10, such solid angles represent the location of the wing collectors210A, 210B.

Preferably, the solid angles in the back quartersphere BQ represent twolocations, preferably substantially identical to one another andpositioned symmetrically with respect to one another and with respect tothe incident plane. In system 10, such solid angles represent thelocation of the back collectors 240A, 240B.

Signal Architecture, Contd.

The communication network that is represented in FIG. 46 as switch 691could be any suitable communication system, such as an Ethernet™communication system or, preferably, a Serial PCI compatible, switchedinterconnect communication system such as one based on the StarFabric™open interconnect standard, “PICMG 2.17 CompactPCI StarFabricSpecification” (ratified in May 2002).

Turning to FIG. 46, the data acquisition system 54 comprises a pluralityof data acquisition nodes 570 connected by the serial PCI switch 691 toa data reduction system 55 comprising a plurality of data reductionnodes 670. Each data acquisition node 570 is connected to and hasassociated therewith a collection and detection module 200 in theoptical collection and detection subsystem 7. Each light channelcollection and detection module 560 and dark channel collection anddetection module 200 has an output that is connected through anassociated amplifier 693 to the input of a filtering unit comprising anA/D 572 (also known herein as an A/D converter 572) and a Processingunit (PUs) 574. The processing unit 574 comprises a microprocessor or,alternatively, a field programmable gate array (FPGA), and providedigital filtering and have outputs to the serial PCI switch 691.

The light channel collection and detection module 560 has associatedtherewith elements of the quad cell detector 258, specifically theextinction element, radial element, tangential element, andheight/reflected power element. The dark channel collectors 300 compriseback left collection and detection module 340B, back right collectionand detection module 340A, center collection and detection module 320,and front collection and detection module 330 s, and further compriseright wing collection and detection module 310A and left wing collectionand detection module 310B, each of which can be operated in P-polarizedand S-polarized configurations.

The data reduction subsystem 55 comprises a selected number of datareduction modules 670, also called data reduction nodes 670. In theillustrative but not necessarily preferred embodiment, the datareduction nodes 670 comprise dual PC-type processors in the workstationclass, specifically having a 64-bit architecture. The nodes 670 couldalso comprise a series of standard rack-mounted computers (bladeprocessors). Each data reduction module 670 has an input that isconnected to the serial PCI switch 691. As mentioned above and describedin more detail below, a data reduction module 670 may be provided foreach of the desired combinations of collection and detection modules 300to be processed into a channel 600 by the surface inspection system 10.

The networking of a plurality of data reduction nodes 670 with aplurality of a data acquisition nodes 570, each of which is dedicated toa collection and detection module 200, provides a signal processingarchitecture in which multiple generic data recipients are available ona peer to peer basis to multiple sensors, thus essentially providingmultiple computing destinations for the collector output. In addition,networking of DANs and DRN allows for simultaneous delivery of identicaldata to multiple destinations, thus allowing for simultaneous usage ofthe data product. For example, the signal processing architecture allowsa user of system 10 to perform “Total Integrated Scatter”-based hazeanalysis in tandem with “Angle-Resolved Scatter”-based haze analysis,both of which are described in further detail below.

The resultant flexibility allows the system 10 to combine any suitablecombination of collectors 300 into a channel 600. The ability to definechannels 600 using any desired set of collectors 300 allows forunprecedented flexibility in surface inspection system output, resultingin improved investigation of surface aberrations.

Referring to FIGS. 48 and 49, there is shown a block diagram showingdata flow in the surface inspection system 10 of the presently preferredyet merely illustrative embodiment of the present invention. An opticsplate 60 has a plurality of collector/detector assemblies 200. In thepreferred embodiment, the optics plate 60 has twelve collector/detectorassemblies 200, a plurality of PMT units 495 and associatedpreamplifiers, one quad cell detector 258 with three output signals(radial, tangential, and extinction) and associated preamplifier, and aLPSD 256 with associated preamplifier with output signals representativeof wafer height changes.

In the embodiment of the present invention that is arranged for theinspection of bare semiconductor wafers, the optics plate 60 has eightPMTs 495, one for each of the center collector/detector assembly 220,front collector/detector assembly 230, and back collector/detectorassemblies 240A, 240B, and two for each wing collector/detector assembly310A, 210B; each wing collector/detector assembly having one PMT 495 forits S-polarized configuration and one for its P-polarized configuration.In the embodiment of the present invention that is arranged for theinspection of semiconductor wafers with transparent films, the opticsplate may have ten PMTs 495 (an additional two on the backcollector/detector assemblies 240A, 240B).

The optics plate 60 is connected to a data acquisition subsystem 54having a gauge synchronization board 186 that is connected to theplurality of data acquisition nodes (DANs) 570. In the presentlypreferred yet merely illustrative embodiment, the gauge synchronizationboard 186 has a 25 MHz master clock and sends synchronization scaninitiation signals to six DANs 570. The DANs 570 comprise a low noisereceiver A/D 572, filters and processing units 574 that as a unit isoperable to perform anti-aliasing filtering, a software-configurablein-scan filtering, analog compression, A/D conversion, digitaldecompression of analog compression function, data decimation, andpreparation of the data for transmission. In an illustrative but notnecessarily preferred embodiment, the filters are a component of theprocessing unit, which comprises a digital signal processor andprogrammable logic such as field programmable gate arrays (FPGA).

The DANs 570 are connected via a switch 691 to the data reductionsubsystem 55, which comprises a plurality of data reduction nodes (DRNs)670. The switch 691 maps output associated with the collector/detectorassemblies 200 to processor inputs in the DRNs 670. In the presentlypreferred yet merely illustrative embodiment, surface inspection system10 comprises seven DRNs 670 that have a combination of hardware andsoftware that is operable to perform linear combining, digitalfiltering, threshold/haze calculation, and data collation andformatting.

The DRNs 670, which comprise a master DRN 672 and at least one slave DRN674, with the master DRN 672 providing set up communications to theslave DRNs 674, are connected via a switch 660 to a system controllerand processing unit 500, which comprises a combination of hardware andsoftware that is operable to provide system control and monitoring,graphics user interface, and defect identification and sizing. Thesystem controller and processing unit 500 is connected to a system I/Ounit 530 that comprises a combination of hardware and software that isoperable to provide subassembly control and monitoring and diagnostics.

The system controller and processing unit 500 is also connected to amotion servo controller 696, which comprises a combination of hardwareand software that is operable to perform stage control and AOD sweepinitiation. The motion servo controller 696 is connected to the gaugesynchronization board 186 in the data acquisition subsystem 54, which isconnected to the digital voltage controlled oscillator DVCO 182 toprovide sweep line control to the AOD 100.

FIG. 50 is a block diagram showing data flow in the DANs570. As notedabove, DANs 570 have a combination of hardware and software that isoperable to perform digital filtering, and data collation andformatting. In the DANs 570, clock, sync and sweep signals aretransmitted to the A/D converters 572 and the Scan line assembly unit578. Also as noted above, raw data is transmitted from thecollector/detector assemblies 200 to the DANs 570, first arriving in theA/D converters 572. The digital data are then transmitted at a rate of400 Mbytes/sec (for 2 channels, 4× oversampling) to a filter/decimationunit 580 for filtering and decimation. The digital data are thentransmitted a rate of 100 Mbytes/sec to a scan line assembly unit 578.

Also as noted above, parameters and commands arrive at the DANs 570 at alow rate from the DRNs 670 via the StarFabric™ connection 691. Thecommands are decoded by a command decoding unit 584, which decodescommands from the signals and sends them to an address distribution unit586 and scan line assembly unit 578.

The decoded commands control the scan line assembly unit 578 inassembling scan lines from the digital data. The assembled digital dataare then transmitted at a rate of 80 Mbytes/sec to a compression unit588 for data compression, and then transmitted out as low voltage datasignals via the Serial PCI switch 691 to the DRNs 670. The addressdistribution unit 586 sends command signals to indicate the DRNdestination of the newly compressed digital data.

FIG. 51 is a block diagram showing the data flow in the Dark ChannelData Reduction Nodes 670, which as described above, comprise acombination of hardware and software that is operable to perform linearcombining, digital filtering, threshold/haze calculation, and datacollation and formatting. The compressed digital data, which isassembled into scan lines, are transmitted at a rate of 80 Mbytes/sec (4channel summing) to a decompression unit 671. The data are decompressedat the data decompression unit 671 and then transmitted at a rate of 160Mbytes/sec to a data combining unit 673 (in which channels are createdas described in accordance with the present invention) and to DCSaturation logic 678, for use in monitoring that will be described inmore detail below.

The combined data are then transmitted at a rate of 40 Mbytes/sec(reduced to single channel) to a cross scan filter unit 676 forcross-scan filtering to be performed on the data in accordance with themethods described in the U.S. Pat. No. 6,529,270, which is herebyincorporated by reference, for background.

The cross-scan filtered data are then transmitted to a thresholding unit680, for use in the thresholding of data as described in detail above,and at a rate of 2 Mbytes/sec to a haze tracking algorithm unit 684 forhaze analysis. In the presently preferred yet merely illustrativeembodiment, an in-scan haze bucketing unit 682 is provided so that thecross-scanned filtered data may be prepared for haze analysis. In thein-scan haze bucketing unit 682, the number of in-scan elements isreduced from 400 to 20, with each surviving element representative of 20original elements and a haze¹ value comprising the mean scatterintensity value from surface roughness scatter, associated with thesurviving element comprises an average of the haze values of the 20original elements associated therewith. In the presently preferred yetmerely illustrative embodiment, signals representative of the survivingelements are then transmitted to the haze tracking algorithm unit 684for haze analysis.

The haze tracking algorithm unit 684 performs haze analysis inaccordance with the methods described in the '701 patent, as well asU.S. Pat. Nos. 6,118,525 and 6,292,259. Haze analysis will be discussedin more detail below.

After the haze tracking algorithm unit 684, the data are transmitted tothe threshold calculation unit 686 for use in determining the thresholdvalue. Thresholds can be calculated from the data using conventionalmethods such as averaging or by actual measuring noise levels andthresholding accordingly. As described in detail above, the thresholdvalue may be calculated using a value γ determined by the accepted falsealarm rate and a background level, of which haze is a part and thecalculation of which the signals representing haze are used by thethreshold calculation unit 686.

The calculated threshold value is then transmitted to the thresholdingunit 680, where it is used in thresholding the data received from thecross-scan filtering unit 676. The thresholded data are then transmittedto the data collation and formatting unit 688, which is described inmore detail below.

After the haze tracking algorithm unit 684, the data also aretransmitted to the line averaging unit 690 in order to performcross-scan averaging of haze data. The haze output is then transmittedto the data collation and formatting unit 688.

The DC saturation logic 678 operates to monitor the extent of saturationof the PMTs 495. When the PMTs 495 receive too much haze signal, theystart to become nonlinear and their size detection accuracy isdiminished. Additionally, excess DC current through the PMT 495 causespremature aging of the detector. Therefore, an upper limit is set on theamount of current that may be obtained from the voltage output of thePMT 495.

If a DRN 670 detects a current signal that is over a user-set limit, itwill monitor the portion of the wafer that has gone over-limit. If itreceives additional signal that is over the user-set limit, the PMT 495will transmit an Abort scan signal, which will end the scan currentlybeing performed. The scan may be re-initiated at a proper detection gainsetting.

The DC saturation logic 678 performs that monitoring using data fromindividual PMTs, and so each PMT 495 is individually tracked forsaturation.

The results of the PMT 495 saturation monitoring are input to the datacollation and formatting unit 688, along with the line averaged data andthresholded data. If no PMT saturation state is found, the data arecollated and formatted and transmitted at a rate of 500 Kbytes/sec tothe system controller and processing unit 500.

Returning to FIG. 49, there is shown a block diagram showingcommunication flow in the surface inspection system 10 of the presentlypreferred yet merely illustrative embodiment of the present invention.The system controller and processing unit 500 communicates via theEthernet Switch 660 with the Gauge Synchronization Board 186, MotionServo Controller 696, Master DRN 672 and Slave DRNs 674.

The system controller 500 sends DVCO set up, AOD Level, Enable scan, andMaster Reset signals to the Gauge Synchronization Board 186, which sendsback Acknowledgement signals and (after a full wafer scan is complete)signals identifying the number of sweeps. The system controller 500sends the following signals to the Motion Servo Controller 696: Normalscan, Slow scan, Servo setup, Tuning commands, Start and Stop command,Stage commands, Trajectory setup and chuck commands. The motion servocontroller 696 in turn sends back acknowledgment, scan position, andMotion status signals.

The system controller 500 sends the following signals to the Master DRN672: DRN Boot, DRN setup, DAN configuration and reset, scan control(such as start, enable, abort, end), acknowledgement. The master DRN 672in turn sends acknowledgement of “Over Threshold and Haze” (OT&H) data,and it sends sensor calibration data signals to the system controller500. The slave DRNs 674 also send acknowledgement, OT&H data, and sensorcalibration data signals to the system controller 500.

The master DRN 672 and slave DRNs 674 are also interconnected by theEthernet switch 660. The master DRN 672 sends the following signals tothe Slave DRNs 674: DRN setup, End scan, Abort scan, and Reset for newapplications. The slave DRNs 674 send acknowledgement signals to themaster DRN 672. The Master DRN 672 and Slave DRNs 674 are connected viaa StarFabric™ bus switch 691 to the DANs. The master DRN sends a switchsetup signal to the StarFabric™ bus switch 691.

The master DRN 672 also sends the following signals to the DANs 570:Switch setup, DAN configuration, Enable scan, End scan, Abort scan,Diagnostic, Operational, DAN bootstrap commands, Switch configuration,Startup to run boot loader. The DANs 570 send detector setup andcalibration signals to the collector/detector assemblies 200, which sendraw data to the DANs 570. The DANs 570 send filtered and decimated datato the master DRN 672 and the slave DRNs 674, and they send Status andAcknowledgement signals to the master DRN 672. The DANs 570 and gaugesynchronization board 186 send low voltage data signals via a backplane960 to each other: the DANs 570 sending DAN Acknowledgement signals andthe gauge synchronization board 186 sending Reset, Clock, and Encodersignals.

The gauge synchronization board 186 and motion servo controller 696communicate via a Differential bus 962, the gauge synchronization board186 sending Status signals and the motion servo controller 696 sendingTrigger and Encoder signals. The gauge synchronization board 186 anddigital voltage controlled oscillator (DVCO″) 182 communicate via anRS-232 bus 964, the gauge synchronization board 186 sending Chirpcommand and Trigger signals and the DVCO 182 sending Acknowledgement andsignals identifying the number of chirps (the DVCO chirps causing an AODsweep).

Signal-To-Noise Ratio (“SNR”) Improvement

Some components of haze actually collected and detected by surfaceinspection systems do not originate on or in wafer under inspection andtherefore have nothing to do with wafer defects. The sensitivity of thesurface inspection system 10 can be strongly influenced by thebackground noise in the system, especially when the system is used todetect extremely small surface characteristics, such as in semiconductorapplications. The relative strength of the desired signal to theundesirable background noise is embodied in the signal-to-noise ratio(“SNR”). In semiconductor applications, Rayleigh scatter from the laserbeam as it propagates through the air within the scanner is an importantsource of background light, and therefore quantum mechanical shot noise.Light reflected internally within the system from other components, forexample, such as light reflected off of apertures or stop also canconstitute unwanted noise. These noise sources are sometimes referred toas “instrument signature” (e.g., scattered light that comes from theinstrument itself, and not from the workpiece under inspection). Inaddition, the electronic components of the surface inspection systemcould provide a certain amount of shot noise.

One approach to improving the SNR is to improve signal strength, forexample, by increasing beam power, frequency, etc. Another approach toSNR improvement involves a reduction in system noise.

In accordance with still further aspects of the invention, a number ofsystems, apparatus and methods are provided for improving SNR. A numberof presently preferred embodiments and method implementations of thesewill now be described. To aid in this description, and to simplify them,they will be described as implemented in system 10. It will beunderstood and appreciated, however, that these aspects of the inventionare not necessarily limited to system and its specific components andimplementations as expressly described herein, and that they may beapplied to other systems and embodiments.

In accordance with the preferred embodiments and implementations ofthese aspects of the invention, system 10 is designed to minimizeinstrument signature and other sources of unwanted background noise. Thepresently preferred embodiments and method implementations have beendesigned using, and based upon, a scatter tolerance budget within thesystem as a whole.

Illumination Absorbing System

To illustrate these aspects and principles of the invention, a surfaceinspection system 10 according to a presently preferred embodiment ofthese aspects of the invention will now be described. The system 10 isuseful for inspecting one or more surfaces of a workpiece is provided.The surface inspection system 10 comprises an illumination subsystem 13that projects a beam to the surface of the workpiece.

In a presently preferred embodiment and method implementation, theillumination subsystem 13 also comprises a beam scanning device, insystem 10 called the beam scanning subsystem 8, which preferablycomprises an acousto-optic deflector such as AOD 100. More preferably,this comprises beam scanning subsystem 8 or module 92 with variable scanspeed AOD 100 as described herein above.

The system according to this aspect of the invention also comprises acollection subsystem for collecting scattered portions of the beamscattered from the surface S of the workpiece W. An illustrative but notnecessarily preferred collection subsystem of the present invention hasbeen described above as the collection subsystem 380 of which theoptical collection and detection subsystem 7 is comprised. Thecollection subsystem 380 comprises collection optics of system 10, whichcomprise components of the collection and detection module 200 abovedescribed, namely, a front collector module 230 with light channelassembly 253, a center collector module 220, a pair of wing collectormodules 210A, 210B, and a pair of back collector modules 240A, 240B, allas described herein above with reference to system 10.

The system according to these aspects of the invention further comprisea processing subsystem 19 operatively coupled to the optical collectionsubsystem 380 for processing signals received from the opticalcollection subsystem 380 to provide information about the surface of theworkpiece.

The illumination subsystem 13 comprises a plurality of lenses or opticalcomponents through which the beam or its component portions pass. Suchlenses or optical components have been described herein as components ofthe beam source subsystem 6 and the beam scanning subsystem 8. Preferredembodiments of these lenses or optical components, such as objectivelens optics 392,have been described herein above.

Individual components of the illumination system 13 through with thebeam passes, and preferably all of such lenses and optical components,have a surface roughness that does not exceed a selected value. In apresently preferred yet merely illustrative embodiment, the surfaceroughness does not exceed about 30 Angstroms; more preferably it doesnot exceed about 5 Angstroms. This limit on surface roughness limitsscatter of the beam and correspondingly maintains a desired amount,preferably a maximum, of the beam energy collimated within the beam.

In addition, a reduction in system noise may be accomplished byproviding the AOD and the collection system with instrument signaturereduction systems employing, for example, combinations of baffles andrelay lenses with glass stops that serve to reduce instrument signature.

In accordance with another aspect of the invention, the illuminationsubsystem 13 comprises an illumination absorbing system 21 comprised ofcomponents of the beam source subsystem 6 and the beam scanningsubsystem 8 described above. As shown in FIG. 81, the illuminationabsorbing system 21 in this aspect of the invention comprises beamsource absorbing system 22 at the beam source subsystem 6 and beam scanabsorbing system 24 in the AOD 100 for absorbing scattered light. Thelaser beam comprises a collimated portion that lies within the main beamand a residual non-collimated portion, for example, that is scattered.The collimated beam portion is reflected off the surface of theworkpiece W to provide the specular beam and the surface scatter that iscollected by the front collector, wing collectors, and/or backcollectors to detect and distinguish surface characteristics, such asdefects. The uncollimated portion of the beam at the illuminationsubsystem typically comprises scattered light not useful in surfacemeasurements. Some of the scatter comes from the Rayleigh scatterassociated with laser beam itself, but most of the scatter comes fromelements of the AOD 100, such as the clean-up polarizing cube 26 seen inFIG. 16 located between the cylinder lens 150 a and the black glassbaffles 114. By providing illumination absorbing system 21, as is donehere, generally undesirable light can be absorbed and removed from thesystem, so that it does not inadvertently enter the collectors andbecome an unwanted part of the measured signal. Scattered light from theAFRU 92 generally appears as increased background signal in the DFRU 811channels.

As implemented in the illumination subsystem 13, the beam scan absorbingsystem 24 comprises means for absorbing light that is not collimated inthe beam, which are located both within and at the output of AOD.

Referring to FIG. 12, which is a top view of the AOD assembly 102, andFIG. 13, which is a side view of the AOD assembly 102, the means forabsorbing light that is not collimated in the beam comprises a series ofapertures, baffles and threads to absorb undesired scatter. Theapertures are sized to allow the collimated portion of the laser beam topass but operate as baffles for collecting scatter.

In the presently preferred yet merely illustrative embodiment andreferring to FIGS. 16-17, the series of apertures, baffles and threadscomprises the following:

-   -   Aperture 110 is located at the opening of the AOD assembly 102        and is sized to allow the passing of the incoming laser beam.    -   Aperture 111 is located at the input of the AOD 100.    -   Aperture 113 is located after the AOD 100.    -   Aperture 117 is located at the sliding plate 158 for moving the        variable speed assembly cylindrical lens 150A or 150B into        position.    -   Aperture 119 is located at the input of the AOD beam splitting        cube 26 (called the polarization clean-up cube 26 above)(which,        being oriented at P polarization itself, itself substantially        reduces the S-polarized stray light from the AOD).    -   Aperture 121 is located after the beam splitting cube 26.    -   Baffles 114, are located after aperture 121 and are preferably        comprised of one or more pieces of black glass (e.g. Schott UGI)        that are disposed at the Brewster angle for the particular        absorbing glass type. These absorb the zero order laser beam        when the AOD 100 is not on. When the AOD is on, the laser beam        is diffracted away from the baffles 114. However, the baffles        114 absorb the residual stray light scatter generated by the AOD        100, cylinder lenses 150A, 150B, and cube 26 outside the laser        beam scan aperture region.    -   Aperture 116 is an adjustable aperture and is positioned        immediately before the drive of the wave plate 118.    -   Aperture 123 is positioned at the interface where the beam comes        into the AOD snout.    -   Aperture 125 is located before the telecentric lens 120.    -   Threads 122, which are located after the telecentric lens 120        and within the AOD snout 124, operate as a baffle structure for        collecting scatter.    -   Aperture 126 is located at the end of the AOD snout.

Any further residual scatter then goes through the light channelspecular beam aperture 251 and is absorbed either by the absorbingattenuator 242 in the light channel assembly 253 or by the Lyot stop770, both of which are components of the collector/detector absorbingmeans 270 described below.

Light Channel Absorbing Means

In accordance with still another aspect of the invention, a lightchannel absorbing means 252 is provided at the light channel assembly253 for attenuating the light that propagates into the light channel.

As was described herein above, and as can be seen in FIG. 29, the lightchannel assembly 253 of system 10 uses a compact optomechanical designthat splits the incident beam into two beams, directing them into thequad cell detector 258 and light position sensitive detector (LPSD) 256.As shown in FIG. 85, the collector/detector absorbing means 270 has alight channel absorbing means 252 to transmit reflected light from thewafer into the light channel assembly. In the illustrated but notnecessarily preferred embodiment, the light channel absorbing means 252comprises an absorbing attenuator (OD=2.0, typical) 242.

The attenuator 242 comprises an absorbing glass, for example, blackglass, which further minimizes the amount of light that is reflected.Light that is incident on the attenuator in this embodiment andimplementation is predominantly P-polarized. The attenuator 242 isoriented at the Brewster angle to maximize the amount of light thattravels through the attenuator glass. The attenuator 242 does not have acoating of any type, in its preferred embodiment.

Light that is scattered from the mirror assemblies must pass backthrough the attenuator 242 in order to reach the wafer surface,therefore the light channel is optically isolated from the detection andcollection subsystem. This can be an important noise attenuationapproach given that the optical power entering the light channel can bemany orders of magnitude higher than the amount of light that iscollected by the collectors that are used to form the dark channel.

Lyot Stop

In accordance with yet another aspect of the invention, a surfaceinspection system 10 is provided, as generally described herein above,but which further comprises a collector/detector absorbing means 270also having a Lyot stop 770. As seen in FIG., 20, the Lyot stop 770 islocated above the specular beam tube of the light channel assembly 253and within the area containing the baffles B2 in the collector modulebarrel housing 394 of the front collector 330. FIG. 56 illustrates theplacement of the Lyot stop 770 relative to the lenses L1, L2 and thetelecentric lens 120. FIG. 56 is a beam trace of light emanating fromthe telecentric lens 120 to the wafer surface S at the telecentric plane498 and scattering into the lenses L1, L2. Location I1 is the imageplane for light emanating from the AOD 100. Location I2 (between thelocation I1 and the lens L2) is the image plane of light emanating fromthe telecentric lens 120. The Lyot stop 770 is positioned betweenlocations I1, I2.

The Lyot stop 770 is cup shaped. Preferably, it is formed of anodizedaluminum and sized so that, in image space, the length of the Lyot stop770 is longitudinally the length of the AFRU 92 optical system. In thepresent preferred yet merely illustrative embodiment, the Lyot stop 770is sized and shaped so that scatter from the AFRU telecentric lens 120is focused toward the back of the Lyot stop 770, and scatter from theAOD 100 is substantially focused into the front of the Lyot stop 770.

In addition, the Lyot stop 770 is also angularly separated from thespecular beam to provide improved separation of the AFRU 92 scatteredlight from the scattered light that propagates into the front collector3301 and light channel assembly 253.

As shown in FIG. 25, the collector/detector absorbing means 270 alsocomprises a series of baffles and glare stops to absorb undesiredscatter. As shown in FIG. 25, baffles B2 in the collection opticssubassembly 390 are provided above the objective lens L2 to minimizestray off-axis light. Stray light that passes by the baffles B2 will befurther reduced by slit 396.

Detector Slit Tracking

As noted above, and referring to FIG. 25, the collectors have a slit 396through which the objective lenses L1, L2 focus the incoming photons.The slit 396 operates as a field stop to absorb scatter outside theregion illuminated by the laser spot. The width of the slit 396 isselected to so that the slit 396 is at least wide enough accommodate theimaged spot size on the wafer W.

In one embodiment, the width of the slit 396 is oversized to adjust formechanical tolerances due to wafer height variations. As the waferheight changes due to wafer bow and warp, the intersection point wherethe laser spot and the wafer meet varies. This movement of theintersection point causes the scanned spot on the wafer to move fromside to side as the wafer spins. The width of the slit 396 is selectedto be oversized to allow the imaged spot to pass through the field stopas the local wafer height changes during the scan. In anotherembodiment, in order to minimize the Rayleigh scatter that an oversizedslit 396 would allow into the collector 300, the width of the slit 396is matched to the beam size on the wafer W, and wafer tracking means,comprising a tracker 398 formed of known hardware and software elements,is provided to move the slit 396 to accommodate changes in the waferheight. The tracking means 398 comprises any suitable mechanism to movethe slit 396 in any conventional way, such as linear stages or PZT orthe like. For example, the tracking means 398 could comprise a controlsystem that uses the signal from the light channel LPSD 256 to sense thelocal height of the wafer, and thereby move the slits 396 in eachcollector 300 to compensate for the associated imaged spot movement.

Stray light that passes through the slit 396 will be further reduced bythe glare stops G1, G2, G3 that are, respectively, located immediatelybefore the collimating relay optics lens L3, after the collimating relayoptics lens L3, and before the relay optics lens L4. Finally, anyresidual stray scatter light will be minimized by the field stop F2,immediately before the photocathode. In the two collector (dual PMT 495)embodiment of the current invention, the field stop F2 comprises a slit(such as detector slit 496 in FIG. 26). In the embodiment featuring a 90degree collector, the field stop F2 comprises a hole.

Beam Source Pre-Alignment System

In accordance with another aspect of the invention, a method and systemis provided for assembling a surface inspection system 10. This assemblymay occur as part of a new system assembly, as part of a systemmaintenance or repair effort, or the like. A presently preferredimplementation of this method and system will now be described. Tosimplify the description .and illustration, this preferred method andsystem implementation will be described with respect to system 10. Itwill be understood and appreciated, however, that neither the method northe system is limited to this specific system embodiment, and thateither the method or the system may be implemented using otherembodiments, apparatus and implementations.

In accordance with the preferred assembly method, the beam source module70 is aligned so that the laser beam is directed to the pointingposition with 50 microradian accuracy. To facilitate this task, a beamsource pre-alignment system 824 is provided. FIGS. 73 and 74 are blockdiagrams showing implementations of the pre-alignment methodcontemplated by the present invention. FIG. 73 shows an implementationof the beam source pre-alignment system 824, which comprises a base,such as beam source module base plate 76, apertures, such as aperture924, 925, a reverse telescope 826, and alignment detector 838,also knownas photodetector 838 (a digital camera or other optical-to-electricalconversion means). The pre-alignment system 824 also optionally butpreferably includes a display 922, and/or a signal processor 926 coupledto the photodetector 838, for processing and/or displaying the beamalignment. The beam source pre-alignment system 824 further comprises aholding device 920, also known as a beam source module mounting pad 920,such as a jig that is identical with or similar to the base 11 to whichthe beam source and scanning mechanism will be attached.

In the illustrative yet not necessarily preferred embodiment of the beamsource pre-alignment system and method according to this aspect of theinvention, and with reference to the drawing figures, particularly FIG.13 and FIG. 73, the method of pre-alignment may be performed using theholding device 920 to hold the beam source module base plate 76 in theposition at which the beam source and scanning mechanism will beattached to the optics base plate 60. As is known by persons of ordinaryskill in the art, the turning mirrors 82, 84 may be used to adjust theincident beam vector IB, with the reverse telescope 826 being used tomagnify any small change in the position of the incident beam vector IBat the aperture 924. The photodetector 838 is operated to detect thecurrent pointing position of the laser after the aperture 926, and itsoutput is sent to the signal processor 926, which identifies the currentpointing position. The video display 20 to which the photodetector 838is coupled displays the current alignment of the incident beam vectorIB.

As part of the preferred assembly method for system, the beam scanningmodule 92 is pre-aligned to the pointing position as well. This isfacilitated using a beam scanning pre-alignment system 822, a presentlypreferred embodiment of which is shown in the drawing figures,particularly in FIG. 11 and 74, and described above with reference tothe design of the surface inspection system 10, in which the beamscanning module 92 is mounted to the beam source module 70 by operationof pins 96 mating a corresponding plurality of pinholes 94 in the beamsource module base plate 76, and in which the beam scanning module 92 ismounted to the base 11 by operation of pins 128 on the bottom surface ofbeam scanning module base plate 90 mating a corresponding plurality ofpinholes 130 in the optics base plate 60.

Inspection Method

In accordance with another aspect of the invention, methods are providedfor inspecting a surface of a workpiece, as noted herein above.Presently preferred implementations of these methods will now bedescribed. For ease and simplicity of illustration, these preferredmethod implementations will be described in conjunction with the system10 according to a presently preferred embodiment of the invention as ithas been described herein above. It should be understood andappreciated, however, that these preferred method implementations arenot necessarily limited to the systems, subsystems, components andassemblies as described herein with respect to the preferred embodiment.

In accordance with this aspect of the invention, a method is providedfor inspecting a surface of a workpiece. The workpiece and the surfaceto be inspected are as have been described herein above. In thispreferred but illustrative implementation of the method, the workpiece Wcomprises an unpatterned semiconductor wafer, and the surface Scomprises one of the planar surfaces of the wafer upon which dies willbe formed in subsequent processing.

In accordance with this preferred method, the wafer is positioned forinspection, preferably by using a robotic wafer handling subsystem suchas workpiece movement subsystem 15 to place the wafer on inspectiontable 9.

This preferred method comprises providing an incident beam and scanningthe beam on the surface of the workpiece so that a portion of the beamis reflected along a light channel axis LC in a front quartersphere FQ.The method further preferably but optionally comprises providing a lightchannel collection and detection assembly 560, which is centered uponlight channel axis LC. The channel developed from the output of theassembly 560, referred to herein as the light channel 650, receives thebeam reflected from the workpiece surface S.

The method also comprises collecting a scattered portion of the incidentbeam at one or more wing collectors disposed in the front quartersphereFQ, outside the incident plane, and at a null or a local minimum, insurface roughness scatter relative to defect scatter, for example, froma defect perspective, at a maximum in the signal to noise ratio ofdefect scatter to surface roughness scatter when the incident beam is Ppolarized, or, from a surface roughness scatter perspective, when thesurface roughness is at a relative minimum (BRDF_(MIN)) of the BRDF whenthe incident beam is P polarized.

The method further comprises collecting scattered portions of theincident beam at a plurality of back collectors disposed in the backquartersphere BQ.

In addition, the method comprises detecting the collected portions ofthe incident beam and generating signals in response.

The method further comprises collecting scattered portions of theincident beam at a plurality of collectors 300 and identifying defectsusing signals from selected combinations of collectors 300.

The method further comprises collecting scattered portions of theincident beam at a plurality of collectors 300, comprising wingcollectors 340 and dual back collectors 310, and classifying defects ona workpiece W based on differences in the angular distribution of thelight scattered from the workpiece.

In addition, the method comprises collecting angular components ofscatter light that is collected by multiple collectors 300 arranged tocollect light from multiple conical regions above a surface S in thelaser-based surface inspection system 10, and using the angularcomponents to facilitate defect classification.

The method further comprises comparing the amount of light collected byone or a combination of collectors to the amount of light collected byone or more of the other collectors 300.

In addition, the method comprises comparing the amount of lightcollected by one or a combination of collectors 300 to the amount oflight collected by one or more of the other collectors 300.

In the context of semiconductor wafer or chip inspection, and in likeworkpieces, a considerable fraction of the beam energy that is scatteredfrom the workpiece surface is distributed outside the plane of incidenceof the beam. The scattering of energy from a particle defect on asemiconductor wafer is known. It includes energy predominantlydistributed in an annulus. For surface inspection systems wherein thedetector lens assemblies are arranged solely in the plane of incidence,some of this energy may be missed. Inclusion of back collector anddetector assemblies 240 therefore improve the ability of the system totake advantage of this energy to improve signal strength for defects.Moving the back collector detector assemblies 240 location to a positionin the back quartersphere BQ that is 45° out of the plane of incidenceimproves back collector detection of polystyrene latex spheres (“PSL”s).When the back collector 240 was in the plane of incidence (as shown inU.S. Pat. No. 5,712,701), Rayleigh air scatter from the laser beam wascoupled into the detector, thereby raising the background level andreducing the signal-to-noise ratio (SNR) of this collector. By movingthe detector out of plane, less Rayleigh air scatter was coupled intothe collector while the scattered light detected from particles on thewafer surface was nearly the same. As a result, the SNR substantiallyimproved. A back collector orientation with an azimuthal angle of 235degrees (0 degrees is outgoing laser beam propagation direction) and ameridional (or elevation) angle of 53 degrees is used in known priorsystems.

Unfortunately, the single out-of-plane detector scheme described in U.S.Pat. No. 5,712,701 has some unfortunate drawbacks. Silicon wafers arenormally polished with polishing pads to generate an extremely smoothsurface. The pads also produce fine structure in the surface thatbehaves like a grating mirror when illuminated with a laser beam. As thewafer is scanned, the laser spot is diffracted by the grating surface inthe direction perpendicular to the polisher-induced “groove” structure.The fundamental direction of the diffracted light changes as the waferrotates, therefore the background scatter into the back collector varieswith the rotation angle, producing a haze map with excessive amplitudevariation, or “bow tie” effect. Users who want to sort wafers by surfaceroughness find it difficult to do so because of this effect. They wouldprefer a surface roughness map that has minimal “bow tie” effect and ismore representative of the Total Integrated Scatter (TIS) from thewafer. In addition, the scanner exhibits lower sensitivity in theregions where the background level is higher, creating a sensitivityvariation around the wafer.

It must also be noted that some kinds of defects may be undetectablewhen detection occurs in only one plane. Scratches are an example of adefect that falls into this category. A scratch having an orientationthat is perpendicular to the AOD scan direction is detectable using afront collector if no edge exclusion mask is present. However, as thewafer rotates, the orientation of the scratch changes with respect tothe AOD scan direction. When the orientation of the scratch is 45degrees with respect to the AOD scan direction, much of the scratchsignal is no longer collectable by any detectors. By positioningcollectors outside of the plane of incidence in order to form “wing”channels, signal from scratches oriented 45 degrees with respect to theAOD scan direction can be detected, thereby improving complete scratchdetection throughout the length of the scratch at various orientationsto the incident beam.

It should be noted that methods of scatter detection that use a TotalIntegrated Scatter (TIS) collector system will not be as sensitive tothese kinds of scratches since they inherently collect scatter from alldirections at once. Since the scatter from the scratches is verydirectional in nature, these scratch defects will be “washed out” by thebackground signal from regions of the collection hemisphere where thereis no scratch signal, thereby reducing the effective sensitivity of thesystem to scratches. By using separate angle-resolved detectors, thescratch signal can be localized to a particular detector and detectedindependently from the other collectors, thus avoiding the effectivereduction of scratch signal that results from averaging signals frommultiple collectors, some of which have collected scatter representativeof workpiece locations where no scratch is present.

As described in the Stover reference , incident laser light is scattered(or diffracted) from the surface in relation to the surface structurespatial frequency content of the surface roughness. The 2D gratingequation relates the scattering angle (in spherical coordinates) to aspecific 2D surface structure spatial frequency coordinate. The AngleResolved Scatter (ARS) architecture described in the '701 patent, aswell as U.S. Pat. Nos. 6,118,525 and 6,292,259 utilizes collectionoptics to collect scatter from specific angular regions of thecollection sphere. These angular regions correspond to regions of the 2Dsurface structure spatial frequency spectrum.

Typically, surface inspection systems employ only an out-of-plane backcollector to provide scatter information that is strictly in theout-of-plane or cross-plane surface structure spatial frequency region.In such systems, defects producing non-symmetrical scatter distributionscan scatter light into space above the workpiece associated with surfacestructure spatial frequency ranges where no collection optics arelocated. In addition, non-symmetrical background surface structurecauses the surface roughness scatter to change intensity and directionas the in-scan and cross-scan directions change with respect to thewafer surface as the wafer is rotated during the spiral scan.

The surface spatial structure frequency plot for the surface inspectionsystem 10 of the present invention is shown in FIG. 52. As can bereadily seen, the new design provides more complete collection ofscattered light associated with the surface structure spatial frequencyspectrum with the addition of channels. This enables simultaneousmeasurement of the Total Integrated Scatter (TIS) from the wafer and theAngle-Resolved distribution of the Scatter (ARS). By combining both TISand ARS in one system, the scanner achieves both improved detectionsensitivity and defect classification capability.

One may use wing collectors 310A, 310B, but no back collectors 340A,340B, or back collectors 340A, 340B and no wing collectors 310A, 310B.Preferably both are used.

In order to determine the geometry of a defect, prior art surfaceinspection systems collected scattered portions of the incident beam ata plurality of detectors, applied a threshold separately to each, thenevaluated the results for defect classification.

Signal Analysis

Channel Definition, Contd.

In accordance with the current invention, a system and method isprovided for detecting the presence of defects by collecting scatteredportions of the incident beam at a plurality of collectors 300 andidentifying defects using signals from selected combinations ofcollectors 300.

In one embodiment of the invention, the method, hereinafter known as thecombined scatter method or CFT method 812, further comprises the step860 of combining output from selected collectors 300, the step 870 offiltering and then threshold testing. In another embodiment, the method,hereinafter known as the individual collector processing method or FTCmethod 814, further comprises the step 870 of filtering output fromselected collectors 300 and threshold testing, and then the step 860 ofcombining the resultant output. Other methods of collector combining areenvisioned in the scope of the present invention, and some of them willbe discussed as examples in greater detail below.

The collectors 300 in the surface inspection system 10 of the presentinvention, shown in block diagram form in FIG. 39, comprise frontcollector 330, center collector 320, a pair of wing collectors 310A,310B, each operable in P or S orientation, and a pair of back collectors340A, 340B. The collectors 300 are positioned to collect scattered lightcomponents in a significant amount of the region in which scatter fromdefects are primarily distributed. Light detected by the variouscollectors 300 signifies a defect and surface roughness in or on thesurface S of the workpiece W. Signals from the collectors 300 areselectively combinable using hardware and software elements to enabledetection and classification of defects in the presence of noise.

FIG. 40 is a block diagram showing an embodiment of the currentinvention of a method for detecting the presence of defects bycollecting scattered portions of the incident beam at a plurality ofcollector detector assemblies 200 and identifying defects using signalsfrom selected combinations of collector detector assemblies 200.Specifically, FIG. 40 shows one method for formation of a sphericaldefect channel chord 815, which is particularly useful in identifyingsmall spherical objects such as PSLs and defects with like geometries.The method illustrated comprises the combined scatter method ofcombining output from selected collectors (CFT method 812), having thestep 860 of combining output from selected collectors, and the step 870of filtering and then threshold testing. Chords and the various methodof combining channels, such as the CFT method 812, are described in moredetail below. In the channel combination example illustrated in FIG. 40,the selected combinations of collector detector assemblies 200 comprisethe dual back collector detector assemblies 240A, 240B and wingcollector detector assemblies 210A, 210B configured for P-polarization.

Detection of Defects in the Presence of Noise

Using Thresholding Based on Modeling Detector Module

As discussed below, summation of appropriately weighted output fromcollector detector assemblies 200 enables optimized detection of smalldefects, while other weighting schemes can optimize for detection ofother defects, such as scratches. The output of the multiple and variouscollector detector assemblies 200, for example, as are presented insystem 10, may be used to determine whether or not scatter from a LightPoint Defect (“LPD”) is as opposed to noise. The following is a methodthat employs multiple and various collector detector assemblies 200, forexample, as are presented in system 10, to detect Light Point Defects(“LPD”) in the presence of noise.

Recognizing that each collector detector assembly 200 will haveassociated with it a constant background light level and a level ofbackground noise, the output of any single detector module 400 in acollector detector assembly 200 in the presence of an LPD is given by:output_(i)=signal_(i) +P _(i)+noise_(i) ,E(noise_(i))=0, E(noise_(i)²)=σ_(i) ², where

-   signal_(i) is the scattering power of a defect at detector i,-   P_(i) is the constant background level; and-   Noise_(i) is the noise associated with the collector, such as shot    noise, electronic noise or pick-up noise.

The output of a detector module 400 if no LPD is present is given by:output_(i) =P _(i)+noise_(i) , E(noise_(i))=0,E(noise_(i) ²)=σ_(i) ².

In accordance with this aspect of the invention, an “optimum” or idealdetector module 400 is constructed, in which a constant expected rate of“false alarms” is established and then the rate of detecting true LPDevents is maximized. It can be demonstrated that the optimum detectormodule 400 is implementable by using a log-likelihood ratio threshold:

${\overset{->}{r} \equiv \begin{bmatrix}{output}_{1} \\{output}_{2} \\\vdots \\{output}_{n}\end{bmatrix}},{{\ln\left( \frac{p\left( {\overset{->}{r}\text{|}{LPD}\mspace{14mu}{present}} \right)}{p\left( {\overset{->}{r}\text{|}{LPD}\mspace{14mu}{not}\mspace{14mu}{present}} \right)} \right)} > \gamma},$with the value γ determined by the accepted false alarm rate.

Whenever the log likelihood ratio exceeds γ, an LPD is declared to bepresent; and as long as the ratio is less than γ, no LPD event isdeclared. Rewriting the equation in terms of the individual detectormodules 400, the log likelihood ratio is given by:

$\begin{matrix}{{\ln\left( \frac{p\left( {\overset{->}{r}\text{|}{LPD}\mspace{14mu}{present}} \right)}{p\left( {\overset{->}{r}\text{|}{LPD}\mspace{14mu}{not}\mspace{14mu}{present}} \right)} \right)} = {{\sum\frac{\left( {{output}_{i} - P_{i}} \right)^{2}}{2\sigma_{i}^{2}}} -}} \\{\sum\frac{\left( {\left( {{output}_{i} - P_{i}} \right) - {signal}_{i}} \right)^{2}}{2\sigma_{i}^{2}}} \\{= {{\sum\frac{\left( {{output}_{i} - P_{i}} \right){signal}_{i}}{\sigma_{i}^{2}}} -}} \\{\sum\frac{{signal}_{i}^{2}}{2\sigma_{i}^{2}}}\end{matrix}$The second expression on the right side of the equation does not dependon measured values, but is solely a function of the noise levels andexpected LPD signal levels of the detector module 400. Therefore, it isconstant and can be pre-computed. The log likelihood ratio test thenbecomes a threshold test of

${\sum\frac{\left( {{output}_{i} - P_{i}} \right){signal}_{i}}{\sigma_{i}^{2}}} > {\gamma + {\sum{\frac{{signal}_{i}^{2}}{2\sigma_{i}^{2}}.}}}$Defining

${G_{i} \equiv \frac{{signal}_{i}}{\sigma_{i}^{2}}},$the log likelihood ratio test becomes a threshold test of

${{{\sum{{output}_{i}G_{i}}} - {\sum{P_{i}G_{i}}}} > {\gamma + \frac{\sum{{signal}_{i}G_{i}}}{2}}};$with G_(i) becoming a weighting value for the output of each collector.Defining

$G_{i} \equiv \frac{{signal}_{i}}{\sigma_{i}^{2}}$provides a set of gains that also comprises the optimum gain weightingfor maximizing the signal-to-noise ratio.

The first summation on the left side of the equation is a weighted sumof the individual collector outputs. The second summation on the leftside of the equation is simply the background level of the weighted sumof the collectors. The right hand side of the inequality is a constantthat may be pre-computed, and comprises the threshold value that, whenexceeded by the weighted output of the collectors, causes an LPD to bedeclared present.

In one embodiment, a system and method for inspecting a surface of aworkpiece by collecting scattered portions of the incident beam at aplurality of collectors and identifying defects using signals fromselected combinations of collectors comprises the step combining theoutput of detector modules 400 associated with a set of collectors,filtering the combined output, and comparing the filtered combinedoutput to a threshold value.

FIGS. 69 and 70 show an illustrative but not necessarily preferredembodiment for a system and method for detecting a light point defect(LPD) greater than a selected size, comprising the following steps:

At Setup time (FIG. 69):

-   -   Step 800: Determine the constant associated with the false alarm        rate γ.    -   Step 802: Track the background levels P_(i) for a selected set        of collectors.    -   Step 804: From the background noise, obtain the noise variance        values σ_(i) ² (including values for Rayleigh noise variance,        non-Gaussian noise including Poisson, speckle noise, and local        haze variation) that are associated with each collector in the        selected set.    -   Step 806: Identify a selected scattering power value signal_(i)        associated with each collector in the selected set, to obtain        the scattering power of an LPD of a selected size s at each        collector in the selected set.    -   Step 808: Derive collector weighting coefficients comprising a        gain of

${G_{i} \equiv \frac{{signal}_{i}}{\sigma_{i}^{2}}},$

-   -    associated with each collector in the selected set.,    -   Step 809: Divide the summed weighted scattering power values by        2

$\left( \frac{\sum{{signal}_{i}G_{i}}}{2} \right)$

-   -    and add γ to obtain the LPD threshold value.

The set-up method shown in FIG. 69 describes a theoretical way to findcollector weighting factors. An alternative set up method comprisesoptimizing the collector gain coefficients (collector weightingcoefficients) for optimal SNR empirically. In the empirical approach,coefficient computation comprises any conventional empirical method,such as 1) collecting raw data from a workpiece or a set of workpieces,2) choosing a set of weighting coefficients, 3) measuring the SNR of thecombined set of raw data, and 4) repeat steps 1) through 3) withdifferent coefficients until the optimal SNR is found.

At Run time (FIG. 70):

-   -   Step 900: Collect the output value output_(i) from the detector        module 400 associated with each collector 300 in the selected        set. Apply the gain G_(i) to each associated output value        output_(i) (either digitally or with an analog circuit) to        obtain the weighted output for each collector in the selected        set.    -   Step 902: Sum the weighted output for each collector in the        selected set (either digitally or with an analog circuit) to        obtain the summed weighted output for the collectors in the        selected set.    -   Step 904: Track the background level of the summed weighted        output to obtain a tracked background level.    -   Step 906: Subtract the tracked background level from the summed        weighted output to obtain a summed weighted        background-independent output value.    -   Step 908: Compare the summed weighted background-independent        output value to the LPD threshold value to determine the        presence or absence of an LPD.

A set of contiguous elements 554 that have over-threshold summedweighted background-independent output values in the output of an AODscan are formed into a channel chord 552. The channel chords 552 soidentified are analyzed by the channel analysis system 520 shown in FIG.46, using currently known techniques to identify defects.

In the above-described combined-scatter method (CFT method 812) fordetection of defects, the output of detector modules 400 associated witha set of collectors 300 is combined, filtered, and compared to athreshold value. In order to obtain optimum detection of LPD events, inthe presently preferred yet merely illustrative embodiment, the selectedset of collectors 300 comprises the set of collectors shown in FIG. 40,namely the dual back collectors 340A, 340B and the P-polarized wingcollectors 310A, 310B.

It should be noted, however, that the invention in its broader aspectsis not limited to combining the collectors 300 using the CFT method asshown in FIG. 40. The present invention contemplates methods of defectdetection in which collector output is combined in other configurationsin order to facilitate collection of other types of defects. Forinstance, the back collectors 340A, 340B could comprise a set ofcollectors to detect certain kinds of substrate defects to which theP-polarized wing collectors are less sensitive.

The combined scatter method for detection of defects, as shown in FIG.40, is particularly useful in improving small particle sensitivity,providing additional information for classification. In addition, ifcombined scatter method is utilized to combine output associated withcollectors as much as possible before input to feature (defect) processmethods, gauge processing requirements will be minimized.

However, the combined scatter method is not preferable for detectingasymmetric defect scatter, such as produced by scratches in theworkpiece surface. As noted above, the invention in its broader aspectsis not limited to the combined scatter method. Another embodiment of thepresent invention comprises a method of defect detection in which theoutput associated with a collector is compared to a threshold valueassociated therewith, and then combined with (similarly thresholdtested) output associated with at least one collector in order tofacilitate detection of other types of defects.

Confidence Level Processing

In accordance with still another aspect of the invention, anothersurface defect detection method for defect detection in the presence ofnoise comprises identifying defects using the statistical significanceof collector output. This method, and more particularly preferredimplementations of the method, can be used with the surface inspectionsystem 10 described herein above, or other systems such as noise-limiteddefect detection systems for which the background noise statistics arewell known. Preferred implementations of this method can substantiallyextend the effective detection sensitivity of the system, enabling usersto make good use of statistically significant data that otherwise mayprovide little benefit or even be discarded in known systems. Preferredimplementations of this method may be used to augment signal processingmethods such as those described in U.S. Pat. No. 6,529,270 (the “'270patent”).

In the preferred embodiments and implementations disclosed in the '270patent, signals from the photomultiplier tube detectors are filtered inthe in-scan and cross-scan directions with a filter that is matched tothe laser spot shape. A threshold is then applied to the filtered2-dimensional data. Values above the threshold are deemed to be “real”defects while those below the threshold are discarded. Morphologicalprocessing is then performed to assess whether the defects are point,area, or line (scratch) defects. The defects are tabulated and displayedon a computer screen. Although the 2-dimensional filtered data exhibitsan optimal signal-to-noise ratio (“SNR”) in a least-squares sense, themethod for identifying defects from this data as described in thepreferred embodiment and implementation of the '270 patent in somecircumstances can be improved. In the preferred embodiment and methodimplementation of that patent, the threshold detector is applied to the2-dimensional filtered data to determine if any defects are present. Dueto the binary characteristics of this threshold detector, points thatlie above the threshold are presented to the customer as real, and areimplicitly assigned a 100% confidence level. All values below thethreshold are ignored by the system, and are essentially assigned aconfidence level of 0%. As a consequence, the threshold value istypically set relatively high with respect to the noise background inorder to minimize false positive events. Because there can be over abillion voltage samples on a 300 mm silicon wafer surface, this impliesthat the threshold should be set at least 6 standard deviations abovethe background noise level (1×10⁻⁹ probability level for a Gaussiandistribution) to ensure that there are no more than a few false events.As a consequence, over 99% of the data is ignored, much of which isstatistically significant and useful. This can cause essentially amismatch between the statistical nature of the data and how it ispresented to the system or method user.

One approach for a system user to accommodate this phenomenon is tolower the threshold below the 1×10⁻⁹ evel in order to see defects ofinterest. Where the false positive events due to background noise aredisplayed with the same statistical significance as the true defectevents, however, this can result in a non-optimal display of “real” and“false” events. Although it is possible to attach a statisticalsignificance and weighting factor to an event, typically the userassumes that a defect is either present or not present, without takinginto account the statistical nature of the underlying data.

This implicates a need for a signal processing system and method thatcalculate the statistical significance of the data, and then faithfullyrepresents this significance to the user. By allowing the user to weightthe bins and displayed defects by the computed statistical significance,data that previously has been discarded can become useful to the user.This can extend the effective detection sensitivity range of the systemand/or method by several nanometers in the context of semiconductorwafer or chip inspection, as will be described herein below.

To illustrate this aspect of the invention, FIG. 57 is a defect map 17depicting 2-dimensional (3.5 mm×5 mm, H×V) scanner data that has beencollected using system 10 as described herein above to inspect apolished, unpatterned silicon wafer. Each pixel in the defect map 17presents an intensity that is representative of a voltage levelcollected at the location on the map that is associated with the pixel.This data has been filtered in the in-scan and cross-scan directions inaccordance with the preferred method described in the '270 patent. Twoback collector maps and two P-polarized wing collector maps wereaveraged together to produce the map 17 shown in FIG. 57. A plurality of50 nm polystyrene latex spheres (“PSLs”) were deposited onto the sectionof the silicon wafer depicted in the map 17 prior to scanning thesurface. These particles can be readily seen in the map 17 as the brightlocations along with the mottled background that is caused by residualshot noise.

FIG. 58 depicts a voltage slice plot 23 that depicts one of these 50 nmparticles. Although the particle peak P in FIG. 58 appears to besubstantially above the noise floor in this particular scan line, thereis a small but finite probability that peaks in the noise floor canoccasionally exceed this level during a complete scan of a wafer.Because there can be on the order of 10⁹ samples across the entiresurface of a 300 mm wafer of this type, the probability of a noisevoltage peak exceeding the height of a voltage peak representative ofthe PSL should be very low (<1×10⁻⁹) to ensure that the background noisepulses can be clearly distinguished from the particle pulses essentiallyall of the time. As a consequence, the detection threshold is usuallyset high enough so that the number of “false particles” on the wafercaused by noise peaks is approximately less than 5. In the case of FIG.57, the threshold should be set at the voltage corresponding to a 50 nmPSL peak in view of the voltage distribution of the background noise toprevent an excessive number of “false particles” from appearing in thedefect map 17 after the threshold level is applied.

FIG. 59 depicts a defect map that was generated from the defect map 17of FIG. 57 by inserting a constant value at each pixel that representeda voltage level that exceeded the voltage threshold value correspondingto a 50 nm PSL. As can be readily seen, nearly all of the informationthat was present in FIG. 57 has been discarded to create the defect map25 in FIG. 59. The voltage signal of the particle signal shown in FIG.58 is barely above the threshold and so would cause the pixel associatedtherewith in the defect map of FIG. 59 to be set to its constant value,while several 50 mn particles appearing in the defect map 17 shown inFIG. 57 have associated therewith voltage signals that fall below thethreshold and therefore do not appear in the defect map 25 of FIG. 59.

It could be expected that, absent the teaching of this aspect of theinvention, if system users set the threshold of a prior known systembased on the plot 23 in FIG. 58, they would probably set it considerablylower than the expected voltage level peak of signals associated with 50nm particles in order to include more of the defects that appear in thedefect map 17 but that do not appear in the defect map 25. However, dueto the statistics of the background noise, too many noise voltage levelpeaks would exceed this threshold when the entire wafer is scanned. Thishas posed problems for users of prior known systems. Since such systemsdo not recognize a gradation of statistical significance, thestatistical significance of the data is misrepresented. If a signalexceeds the preset threshold, the system assumes with 100% confidencethat a defect is present at that location (which is not true). If thesignal level falls below the preset threshold, the system assumes thatthere is no defect (which may or may not be true). What is needed is amethod for calculating the statistical significance of an event, andproperly representing that information to the system user. Because thesurface defect data collected is statistical in nature, what is furtherneeded is to provide a processing system for a surface inspection systemthat operates on surface defect data on a statistical basis. Thestatistical significance of the surface defect data can be used in thebinning recipe calculation to determine whether the wafer has passed orfailed inspection. A presently preferred implementation of this process,which will be referred to herein as “Confidence Level DetectionProcessing Method 502,” will now be described.

The defect map 17 depicted in FIG. 57 is a 2-dimensional voltage mapthat contains representations of both background noise voltage signalsand defect voltage signals from a portion of a surface S of a workpieceW. Scatter from the surface structure (measured as haze) appears as aconstant background level. Scatter from defects (particles, scratches,epitaxial spikes, COPs, etc.) is added to this micro-roughnessbackground, producing small, localized peaks in the voltage map 17.Since the detected power levels in semiconductor wafer inspectionapplications typically are very low (picoWatt to nanoWatt range), themap 17 is dominated by shot (quantum-mechanical) noise. Shot noiseexhibits a Poisson probability distribution, but can be accuratelyapproximated by a Gaussian distribution if the number of detectedphotoelectron events within the effective integration time (orbandwidth) of the detection system exceeds ˜30 photoelectrons. Forsystem 10 as described herein, this condition is normally met after2-dimensional filtering is performed on the voltage maps 17, thereforethe underlying background noise distribution of FIG. 57 can be assumedto be Gaussian.

The measured distribution (black points) and underlying Gaussianbackground noise distribution (X marks) of voltage values associatedwith the region locations represented by pixels in the defect map ofFIG. 57 are shown in the plots 542, 544, 546 of, respectively, FIGS. 60,61, 62, each of which present a distribution of voltage levels. Theblack points were calculated by counting the number of voltage valueswithin a ±100 microvolt range around each selected voltage level. Theresulting measured voltage count curve 504 is the measured probabilitydistribution for the defect map of FIG. 57. The underlying Gaussianbackground noise probability distribution was calculated by fitting aGaussian to the lower (left) half of the measured probabilitydistribution where the defect signals are not present; it is representedby the noise probability curve 506. As can be seen in the plot 542 ofFIG. 60, there is significant signal content above the Gaussianbackground noise probability distribution for voltages greater thanabout 41 mV. FIG. 61 is a plot 544 that gives an expanded view of aportion of the measured probability distribution and Gaussianprobability distribution, as represented by curves 504, 506 of the plot542 of FIG. 60, showing a “hump” near 45-46 mV. This hump is produced bythe 50 nm PSLs on the surface S of the workpiece W represented by thedefect map 17 of FIG. 57. The hump is shown even more clearly on theplot 546 of FIG. 62, which depicts an even more expanded scale view ofthe curves 504, 506 shown in the plot 544 of FIG. 61.

In some prior known systems and methods, the threshold level is set atthe voltage corresponding to ˜50 nm (6 standard deviations above thebackground noise mean), or 46 mV, as shown in FIG. 59. This levelcorresponds to the far right edge of the plot in FIG. 60. Although thisthreshold level ensures that there are minimal false detected events onthe wafer, it is readily apparent in FIGS. 60-62 that interesting databetween 42 and 46 mV would be discarded. The differences between theblack points on the measured voltage count curve 504 and X marks on thenoise probability curve 506 in FIGS. 60-62 indicate that there arethousands of voltage sample values above the underlying background levelin this voltage range that are statistically relevant. The presentlypreferred method implementation provides a means to beneficially utilizethis data.

As indicated above, the difference between the measured curve 504 (blackpoints) and noise curve 506 (X marks) in FIGS. 60-62 representsmeaningful signal content that is present in the map 17 of FIG. 57. Tofurther quantify this, a Confidence Level Factor (CLF) can be defined asfollows:

$\begin{matrix}{{{CLF}\lbrack V\rbrack} = \frac{{H\lbrack V\rbrack} - {B\lbrack V\rbrack}}{H\lbrack V\rbrack}} & (1)\end{matrix}$where

-   -   CLF[V]=Confidence Level Factor,    -   H[V]=the count of voltage values that are measured within a        pre-specified voltage range centered on a selected voltage V,        within a selected region of a workpiece surface,    -   B[V]=the count of voltage values in H[V} that are associated or        expected to be associated with background noise.

When the CLF is multiplied by 100%, it is referred to as the ConfidenceLevel (CL). If the calculated background noise distribution is verysmall, the CL will be approximately 100%. As the noise level increasesrelative to the measured signal content, the CL will decrease.

Note that the CLF is spatially dependent. For example, consider the CLFcurve 508 shown in the plot 514 of FIG. 63, which was computed for thesample set of voltages that are represented in the map 17 of FIG. 57.This CLF curve 508 will not necessarily be the same as one produced forthe set of voltages associated with a scan at another part of the wafer,where the background and defect-induced voltage distributions may bedifferent from those shown in the defect map 17 of FIG. 57. In the caseof FIG. 57, there are numerous 50 nm PSLs present on the regionrepresented by the map 17, therefore the CL for the detection of thesedefects is >99%. The CL would be lower in a wafer region that containsvery few 50 nm particles and a higher micro-roughness background. Thismeans that the effective sensitivity (and statistical significance ofdefects) will vary across the wafer W as the local conditions changeduring the scan. Instead of using a fixed sensitivity threshold toselect and bin, or categorize, defects, the statistical CL can now beused to perform this function.

The CLF curve 508 shown in the plot 514 of FIG. 63 shows the ConfidenceLevel Factor as a function of voltage generated by directly applyingEquation 1 to the data set represented by the measured voltage countcurve 504 and the noise probability curve 506 shown in FIGS. 60-62. TheCLF curve 508 in FIG. 63 is accurate above 0.04V and below 0.05V. Thepart of the CLF curve 508 in the 0.035-0.04V range is normally ignoredbecause non-zero CLFs in the 0.035-0.04V range in FIG. 63 represent anexpected mismatch between the Gaussian background noise fit and themeasured data. The CLFs in the 0.035-0.04V range are thereforeartificially set to zero. Similarly, the CLFs above 0.05V areartificially set to one. The CLF for the data represented by map 17increases from zero at 0.04V to 1 as the voltage increases. Above 0.05V,the CLF for the data represented by map 17 decreases in an intermittentmanner due to the fact that the number of signal events decrease withincreasing voltage and there is minimal signal after a certain voltagelevel. Therefore the CLF drops to zero at numerous voltage levels inthis region. In this particular data set (map 17), there is minimalsignal above 0.05V.

If a deposition of larger particles were present, the values in thisregion would be near 1. Bearing in mind that any voltage value above0.046V is 6 standard deviations above the background noise mean (theusual threshold used in the single-threshold technique), it is possibleto set the region above 0.046V in the plot 514 of FIG. 63 to 1 withoutintroducing any more false defects than the single-threshold techniquewould produce.

By setting the lower region of the plot 514 of FIG. 63 to 0, the upperregion to 1, and employing a standard polynomial interpolation method tothe region in between, a cleaner version of the CLF curve 508 isgenerated, as illustrated by the CLF curve 518 in the plot 516 of FIG.64. As can be readily seen in the CLF curve 518, the CLF is negligibleat 41.5 mV (corresponding to a ˜42 nm PSL equivalent peak height) andmonotonically increases to 0.99 at 44 mV (˜48 nm PSL equivalent).

The confidence levels so derived may then be used to assign a confidencelevel to the voltage value that is measured at a location in a region ofa surface under investigation, to identify an extent of confidence thatthe voltage level so measured represents a defect. By mapping eachpotential voltage level to a confidence level, a CLF curve, such ascurves 508, 518 shown in FIG. 63 or 64, can then be used be utilized asa look-up table (“LUT”) to assign confidence levels to voltage values.

The confidence levels could also be used to provide a visualrepresentation of an extent of confidence that a measured voltage levelrepresents a defect at a selected location. For example, in a defect map17 such as the one in FIG. 57, each pixel of the map represents alocation in a region of a surface under investigation. A characteristicof each pixel, such as brightness, could be defined to represent theconfidence level assigned to the voltage level associated with thepixel. For example, pixels associated with voltage values having higherconfidence levels would be brighter, while those associated with voltagevalues with lower confidence levels would be darker. The brighter thepixel on the map, the higher the statistical probability that the defectso represented is real. A defect map 522 in which the extent ofconfidence in an identification of defects is visually represented,known herein as a Confidence Level Map (“CLM”), is shown in FIG. 65.

The Confidence Level Map (CLM) 28 shown in FIG. 65 was achieved byassociating a CLF with each pixel in the map 17 of FIG. 57, using themapping of each potential voltage level to a confidence level expressedby the CLF curve 518 in FIG. 64. Pixels such as pixel 524 having voltagevalues for which the CLF is zero are black, just as they would if asimple threshold were applied. Pixels such as pixel 526 having voltagevalues for which there is a relatively low confidence level are dim,while those pixels such as pixel 528 that have voltage values for whichthere is a relatively high confidence level are bright. Therefore thedefect brightness in FIG. 65 is closely related to the statisticalsignificance of the defect signal. The variation of CLFs is furtherdemonstrated by the slice plot 534 in FIG. 66.

Note that there are considerably more defects displayed in the CLMdefect map 28 of FIG. 65 than in the conventional threshold defect map25 of FIG. 59. The conventional threshold scheme used in FIG. 59effectively uses a “unit step” CLF that is zero up to 46 mV, then 1.0above 46 mV. In contrast, Confidence Level Detection Processing uses aCLF with a gradation of values, thus enabling the use of statisticallysignificant data below 46 mV that would otherwise have been discarded bya “unit step”CLF. The smoothly-varying CLF curve more accuratelyrepresents the statistical significance of each voltage level in the map17 of FIG. 57. For the example shown here, Confidence Level DetectionProcessing effectively extends the defect sensitivity range by severalnanometers.

The CLM defect map 28 in FIG. 65 can be used to generate a defect mapusing standard and known methods of morphological processing. Anaggregate defect confidence level for a defect could be assigned fromthe confidence levels of the set of locations on the workpiece, such asthe wafer, that define the defect, in order to indicate the statisticalsignificance of the defect defined by the set of locations. For example,the surface inspection system could consider a defect to be identifiedat a position on a wafer when a set of locations on the wafer havepositive CLFs associated therewith, are connected together, and have anaspect ratio that is within a certain range. Once a defect is soidentified at a position, the aggregate defect confidence level can beassigned to the position from the set of confidence levels associatedwith the set of locations that define the position. The aggregateconfidence level could be assigned to be the peak confidence value,comprising the greatest value of the confidence levels of the set oflocations that define the position. Alternatively, the aggregate defectconfidence level could comprise the average value of the confidencelevels of the set of locations, preferably weighted by the expectedsample amplitudes, which would be the voltages measured at thelocations. The peak confidence value can be noisy due to shot noise,therefore the preferred aggregate defect confidence level, also called adefect's CLF, is the weighted average value.

Once the aggregate defect confidence level for a defect has beenassigned, the defect can be binned according to its size attributes andconfidence levels of the locations that define the defect. The defectsize can be computed using a peak voltage value comprising the voltagevalue corresponding to the peak confidence level of the locations thatdefine the defect. Other defect sizing techniques that use other valueswithin the defect group may be used as well.

Normally the color of each defect displayed on the display devicecomprises a color that is associated with a bin into which the defect iscategorized. The brightness of the defect displayed on the displaydevice is usually fixed at a specific brightness in the conventionalthreshold technique, whereas, in a system or method for defectidentification incorporating the confidence level detection processingof the present invention, the brightness of the defect so displayed maybe modulated by CLF. This enables the user to visualize the statisticalsignificance of each defect.

The counting of defects can also be modified when using Confidence LevelDetection Processing. Normally a defect is counted within a certaincategory (bin) if, when it is detected, it is found to possess thecharacteristics associated with the bin. Confidence Level DetectionProcessing can further refine the process of bin counting by weightingthe defect count by its CLF. For example, a defect with a CLF of 0.5will have half the weight of a defect with a CLF of ˜1.0. This meansthat it will take twice as many defects with a CLF of 0.5 to equal thenumber of defects with a CLF of 1.0.

By incorporating the CLF into the binning process, the recipes forsorting wafers can utilize the statistical significance of the defectdata. Unlike the conventional method of defect identification usingthreshold processing, defect identification incorporating confidencelevel processing of the present invention allows rejection of wafers ifthere are a large number of very small defects. An aggregate binconfidence level can be assigned for a bin from the confidence levelsassociated with the defects in the bin to indicate the statisticalsignificance of each bin. One such aggregate bin confidence levelcomprises an average bin CL to indicate the average statisticalsignificance of each bin.

With the present invention, confidence level detection processing asdisclosed herein can be used additionally to control the binning anddisplay of data in accordance with the CLF associated with the data. Forexample, data may be processed using CL cutoff limits in order to limitthe identification or display of the number of defects in a region.

FIG. 67 depicts a confidence level map 29 for defect data that has beenprocessed using a CL cutoff limit, specifically a CL cutoff limit of50%. As in the CLM map 28 shown in FIG. 65, the thresholded CLM map 29in FIG. 67 presents the brightness of a pixel according to the CL of thevoltage level associated therewith. However, FIG. 67 differs from FIG.65 in that the brightness of a pixel is presented in the defect map 29of FIG. 67 only if the CL of the associated voltage level is greaterthan 50%.

Thus FIG. 67 depicts a confidence level map comprising a defect map of asurface of a region under inspection, in which the region comprises aplurality of locations, each of which provided with an assignedconfidence level CL_(A), in which the assigned confidence level CL_(A)is set to zero if the voltage level measured thereat has a CL associatedtherewith that is lower than 50%, and with the assigned confidence levelCL_(A) comprising the CL associated therewith if the voltage levelmeasured thereat has a CL associated therewith that is greater than orequal to 50%.

Comparing the CLM map 28 of FIG. 65 with the defect map 17 of FIG. 57,it can be seen that confidence level processing results in a significantfiltering of the amount of background noise in the defect data. Thedefect map 17 depicts voltage values for both background noise anddefect signals, without any ability to distinguish between defect andnoise, while the CLM map 28 shows likely defects by their extent oftheir likelihood. The dimness of display of an unlikely defect indicatesthe likelihood that it constitutes background noise. Thus, confidencelevel processing creates a map that focuses on likely defects.

Comparing the thresholded CLM map 29 of FIG. 67 to the CLM map 28 ofFIG. 65, it can be seen that the use of cut-limits in confidence levelprocessing results in even greater focus on the likelihood of a positionwith high voltage level measurements being a defect. It can be seen thatseveral of the small features near the bottom of the CLM map 28 of FIG.65 have been eliminated from the thresholded CLM map 29 of FIG. 67 as aresult of CL cutoff limits. For example, position 522 is displayed inmap 28 (albeit dimly), but is not displayed in map 29. A position on thethresholded CLM map 29 is considered more significant because it is morelikely that a defect exists at that position. Thus, thresholdedconfidence level processing creates a defect map that focuses on defectsof greater significance.

Comparing the thresholded CLM map 29 of FIG. 67 to the thresholdeddefect map 25 in FIG. 59, it can be seen that map 29 depicts morepositions as being potential defects, but that it also shows by thedimness of such positions the relative unlikehood of their beingdefects. By applying a CL cutoff limit to confidence level processing,the conventional defect size threshold process used to create the defectmap 25 in FIG. 59 is replaced by a statistical CL threshold in map 29.Thus, thresholded confidence level processing exploits the statisticalnature of the data to present defects by their significance.

It is also possible to re-map the CL's to another display look-up table(LUT) to accentuate the presence or absence of various defects withcertain CL ranges. For example, using the example of the thresholded CLMdefect map 29 of FIG. 67, in which pixels representing locations havingconfidence levels lower than 50% are set to zero, the brightness of thepixels representing locations having confidence levels at 50% or greatercan be adjusted to accentuate the differences in their CLs. A 50% CLcould be remapped to 0%; a 100% CL could remain at 100%; and the CLvalues in between could be assigned other intermediate values, forgreater contrast between the 50% and 100% CL's. Thus, the defects sodepicted in the defect map of FIG. 67 could be provided with a widerbrightness range.

Confidence Level Detection Processing does not necessarily improve theunderlying SNR of the system, but it can enable better utilization ofthe available data. If the user were interested in studying individualdefects, then he or she would set the confidence level cutoff limit veryhigh to ensure that each defect is known to be present with highstatistical significance.

It is possible for a few false defects to be presented with a high CLbecause there is a small but non-zero probability that the noise canreach relatively high voltage levels. This effect can be mitigated byapplying a “global” CLF calculation to the entire processed defect wafermap. As described in further detail below, a wafer or a region of awafer region is often sub-divided into regions, with an image of theentire wafer sub-divided into sub-images that are associated with thesub-divided region, to enable defects to be identified on small,manageable quantities of data and to provide good estimates of localbackground noise. When a CLF is calculated for each sub-image, defectsthat are deemed significant in the region local to the sub-image may notbe significant on a global basis when all of the sub-images areconsidered together. A global confidence level image formed by the setof sub-images can be thus be assigned from the set of confidence levelsassociated with the sub-images in the set to indicate the average globalstatistical significance of defects in the image.

In order to take the global defect map results into account, a final CLfor the entire wafer may be displayed to the user, with the final CLcomprising a confidence level that has been modulated by the globalconfidence level across the wafer, thereby reducing the CL for defectsizes that have counts that are similar to the number of false defectsexpected across the entire wafer. The background noise distribution forthe global CLF can be calculated based on the average haze level for theentire wafer. For example, if, at the end of a wafer scan there are 5defects in a particular bin category, and the expected number of falsedefects due to noise across the wafer is 4, the CL's for the defects inthis bin category would be averaged with the global CL of 100%*(5-4)/5,or 20%. A final CL comprising a global confidence level and a localconfidence level, ensures that the statistical significance of the datais weighted on both a local and global basis.

The example shown above demonstrates how Confidence Level DetectionProcessing (CLDP) can be used to detect defects in a statisticallysignificant manner using a fixed map of voltage values. This method canbe applied to virtually any map of data for which the background noisedistribution is known or can be computed, and for which individualdiscrete events are to be detected and identified. Examples of otherapplications include laser defect scanners for other types of materials,digital imaging for defect inspection, and motion processing in highspeed digital video applications.

CLDP is particularly effective if it is applied to a region of a surfacethat has a uniform background noise distribution. The entire surface canbe scanned and stored as one voltage map, then processed using CLDP.This method has two main limitations, however. First, if the surface isa 300 mm silicon wafer surface, this would involve the storage andprocessing of over 10⁹ sample values per detector module 400, placingsubstantial and perhaps excessive demands on the computational hardwareand software required to process this data. Second, if the backgroundlevel varies substantially across the surface, the global CLFs sogenerated will be a poor estimate in the regions that have asignificantly higher or lower background than the global averagebackground level.

By sub-dividing the surface into sub-images, an example of which isshown in FIG. 68 a, the measured distribution and underlying Gaussianbackground noise distributions can be computed for each sub-image,thereby ensuring that the each distribution is a good estimate of thelocal background and enabling CLDP to be performed on small, manageablequantities of data. FIG. 68 b shows how a silicon wafer could be scannedas a series of cylinders, each of which is divided into multiplesub-images that are processed using CLDP. The sub-image technique canalso be applied to a spiral scan pattern, for example, as shown in FIG.68 c.

For each of the scanning geometries shown in FIGS. 68 a-68 c, the numberof scan lines to include within a sub-image is directly limited by theslope of the mean of the background noise, or haze slope. As the maximumexpected haze slope increases, the number of scan lines used in thedistribution calculation should decrease in order to achieve a gooddistribution estimate. A distribution and confidence level map can becomputed for each sub-image. The background distribution used in theconfidence level map can be derived by using a known backgrounddistribution based on calibrated PMT signals, or by performing a fit onthe lower half of each measured distribution. Morphological processingthen may be performed on each confidence level map or sub-image. Somemethods of controlling sensitivity banding may be desirable or required.

Many specific hardware and software implementations of Confidence LevelDetection Processing can be used. As shown in the FIG. 86 and 87, whichis a block diagram of one Confidence Level Detection Processing method502, the method comprises the following steps:

-   -   Step 851: A region of a workpiece to be evaluated for potential        defects is identified.    -   Step 852: A potential signal range is identified for signal        measurements to be obtained from locations in the selected        region. The potential signal range is subdivided into a set of        signal intervals comprising selected signal levels and a        predefined range around each signal level. In an illustrative        but not necessarily preferred embodiment of the current        invention, employing the surface inspection system 10, the        signals comprise voltage signals indicative of photon activity        within a collector 200, with the photon activity resulting from        light scattered from the surface of the region under inspection,        and with the extent of the signal measurement being indicative        of the extent of such photon activity. In addition, in the        illustrative but not necessarily preferred embodiment of the        current invention, the potential signal range comprises a        potential voltage range, which is subdivided into a set of        voltage intervals comprising selected voltage levels V and a        predefined range around each voltage level V. For purposes of        describing the method 502, hereinafter the signals will be        described as voltage signals. It should be understood, though,        the invention should not be limited to such embodiment. In the        embodiment, the voltage levels V could be spaced every 200        microVolts within the potential voltage range, and the        predefined range could comprise ±100 microVolts around each        voltage level.    -   Step 853: Voltage signal measurements are obtained for locations        on the selected region in order to obtain a set of voltage        measurement values for the region.    -   Step 854: The number of voltage measurement values within each        selected voltage interval is counted to obtain a voltage        measurement value count for each selected voltage level V.    -   Step 855: The voltage measurement value counts are sorted into a        voltage measurement distribution function H[V] comprising the        distribution of voltage measurement values counts in the region,        by the selected voltage level V. In a step 856, a plot is        created having a measured voltage curve 504 comprising the        voltage measurement distribution function, with the curve 504        presenting the number of voltage signals at each voltage level.    -   Step 857: The portion of H[V] that likely comprises underlying        background noise is identified by calculating a background noise        probability distribution function B[V] comprising probable        background noise voltage counts by selected voltage levels V.        B[V] is derived by fitting a probability function to a portion        of H[V] in which particle and haze variation effects are        minimal. In such portion of H[V], voltage values are likely to        represent background noise and not workpiece defects.        Preferably, the portion of H[V] used to derive the B[V]        comprises the lower tail of H[V]. Also, preferably, a Gaussian        or Poisson probability function is used. In a step 858, a plot        having a noise probability curve 506 comprising B[V] is created,        with the curve 506 representing the likely number signals that        comprise background noise at a voltage level.    -   Step 859: A Confidence Level Factor (CLF[V]) is defined for each        voltage level V, in order to assign a confidence level to the        voltage value that is measured at a location in a region of a        surface under investigation, to identify an extent of confidence        that the voltage level so measured represents an actual defect.        Confidence Level Factor (CLF[V]) is calculated by:

${{CLF}\lbrack V\rbrack} = \frac{{H\lbrack V\rbrack} - {B\lbrack V\rbrack}}{H\lbrack V\rbrack}$

-   -   -   In an optional Step 860, a Confidence Level (CL[V])            percentage is defined by multiplying CLF[V] by 100.

    -   Step 862: A Confidence Level Factor function is developed and        represented by Confidence Level Factor curve 508, for values of        confidence level factors (CLF[V]) and the voltage levels V with        which they are associated. FIG. 88, which is a block diagram of        further detail for step 862, shows an optional Step 863 of using        CLF cutoff limits in order to limit the identification of        voltage values as potential defects in a region. The cut-off        limits may be developed in a step 864 and step 865. Step 864        involves identifying a minimum potential voltage in the        potential voltage range, below which voltage values are expected        to represent background noise and not actual defects. The CLF[V]        is set to zero for voltage levels at or below the minimum        potential voltage. A step 865 involves identifying a maximum        potential voltage in the potential voltage range; above which        voltage values are expected to represent actual defects and not        background noise. The CLF[V] is set to one for voltage levels at        or above the maximum potential voltage.        -   Finally, the step 862 also comprises an optional Step 867,            which involves employing a standard polynomial interpolation            method to the Confidence Level Factor function to obtain an            interpolated Confidence Level Factor function, represented            by an interpolated CLF curve 518.

    -   Step 868: A CLF curve 508, or the optional interpolated CLF        curve 518, is used as a look-up table to assign confidence        levels to the voltage levels with which the confidence level        factor is associated.

    -   Step 870: A visual display of the region under investigation is        created to visually represent voltage signals measured at        locations in the region and the extent of confidence that the        voltage signals represent actual defects and not background        noise. Preferably, a Confidence Level Map (“CLM”), such as map        522, is created in which each pixel of the map represents a        location in a region of a surface under investigation and a        confidence level representative of an extent of confidence that        an actual defect exists at the location associated with the        pixel.        -   FIG. 89 is a block diagram of further detail for step 870.            It shows a step 871, which involves defining a            characteristic of each pixel, such as brightness, to            represent the confidence level factor CLF[V] assigned to the            voltage level V associated with the voltage measurement            value obtained at the location that is represented by the            pixel.        -   Step 870 also comprise an optional Step 872, which involves            adjusting the brightness range of pixels in the visual            display to accentuate the presence or absence of a potential            defect at the location associated with a pixel and an extent            of confidence that the potential defect represents an actual            defect.        -   Finally, step 870 comprises a step 873, which involves            assigning a zero brightness level to pixels associated with            locations having confidence levels lower than a selected            minimum confidence level, a maximum brightness level to            pixels associated with locations having confidence levels            CLF[V] equal to one, and adjust the brightness levels for            each pixel associated with a location having a confidence            level therebetween to an intermediate pixel brightness level            between the zero brightness level and the maximum brightness            level, with the intermediate pixel brightness level            developed by interpolation from the confidence levels            associated with each pixel associated with a location having            a confidence level therebetween.

    -   Step 874: Potential defects are identified in the region under        investigation and an extent of statistical significance is        associated with each potential defect. FIG. 90 is a block        diagram of further detail for step 874, and shows a step 875        involving, in one embodiment, identifying a potential defect by        identifying a set of contiguous locations in the region under        investigation, with the locations having voltage measurement        values above a defined voltage level and with the set comprising        at least a defined number of contiguous locations.        -   Step 874 also comprises a step 876, which involves computing            the potential defect's size. In one embodiment, the step 876            comprises a step 877, which involves computing the potential            defect's size using a peak voltage value comprising a            voltage measurement value corresponding to the peak            confidence level associated with the locations that define            the potential defect.        -   The step 874 further comprises a step 880, involving            assigning defect confidence level to each potential defect            so identified, and a step 881, which involves developing an            aggregate defect confidence level to the potential defect            from the set of confidence levels associated with the set of            locations that define the potential defect. The aggregate            defect confidence level, and any aggregate confidence level            described herein, could be created in any convention manner,            for example, using step 882A or step 882B. Step 882A            involves defining the aggregate defect confidence level to            comprise the peak confidence value, comprising the greatest            value of the confidence levels associated with the set of            locations that define the potential defect. Alternative Step            882B involves defining the aggregate defect confidence level            to comprise the average value of the confidence levels            associated with the set of locations that define the            potential defect, preferably weighted by the voltages            measured at the locations.

    -   Step 883: The potential defects are sorted into bins according        to size and confidence levels.

    -   Step 884: An extent of statistical significance of potential        defects is associated with each bin by assigning an aggregate        bin confidence level to each bin from the confidence levels        associated with the potential defects sorted into the bin. In a        step 885, an aggregate bin confidence level is assigned to        comprise an average value of the confidence levels associated        with the potential defects in the bin.

    -   Step 886: An extent of statistical significance of potential        defects is associated with the region under investigation by        assigning an aggregate region confidence level to the region        from the confidence levels associated with the bins that        comprise the region.

    -   Step 887: An extent of statistical significance of potential        defects is associated with a workpiece by assigning a “global”        confidence level to the workpiece from the confidence levels        associated with the regions that comprise the workpiece.

    -   Optional Step 890: A region under investigation may be        sub-divided into sub-regions and the confidence level detection        processing method 502 performed on the sub-regions in order to        identify potential defects using a data set of reduced size and        to provide more detailed estimates of local background noise. An        extent of statistical significance of potential defects would be        associated with a sub-region by assigning an aggregate        sub-region confidence level to the sub-region from the        confidence levels associated with the potential defects in the        sub-region.        -   The step 890 is particularly helpful in instances in which            confidence levels, when calculated separately, would differ            significantly across a test surface, such as a workpiece or            a region of a workpiece. For example, a portion of a test            surface could appear to raise issues that are not present in            the rest of the test surface. One area of a workpiece could            have scatter patterns that indicate the high likelihood of            the presence of a defect, while other areas could have            scatter patterns that are more equivocal about the presence            of a defect. Confidence levels that are calculated            separately for areas with differing scatter characteristics            would thus differ significantly. The overall test surface            confidence level, being lowered by the lack of confidence in            the areas with equivocal scatter signal, would not reflect            as strongly as it potentially could the confidence in the            existence of the defect in one of its area. Thus, by            allowing a test surface to be subdivided, a “global”            confidence level may be modulated across the test surface.        -   In a step 891, an extent of statistical significance of            potential defects could be associated with the region            subdivided in step 890 by assigning an aggregate global            region confidence level to the region from the confidence            levels associated with the sub-regions.

Method for Combining Collector Output

FIG. 41 is a block diagram showing the embodiment of the FTC method 814for detecting the presence of defects by collecting scattered portionsof the incident beam at a plurality of collectors and identifyingdefects using signals from selected collectors, comprises the step 870of filtering and threshold testing, and then the step 860 of combiningoutput associated with selected collectors 300.

One further embodiment comprises combining thresholded output associatedwith the entire set of collectors in the surface inspection system.Another further embodiment comprises combining thresholded output from aselected set of collectors.

The individual collector processing method of FTC method 814 is usefulin detecting and classifying asymmetric scatter, e.g. defects on roughersilicon surfaces and “flat” defects. More particularly, it is usefull indetecting and classifying defects of various spatial frequencies andgeometries that scatter with the symmetry of small particles.

As shown in FIG. 41, a method of using independent or individualprocessing of collector output to analyze defects, which employs theindividual collector processing method or FTC method 814, comprises thefollowing steps:

-   -   Step 832: Define a channel 600 by identifying a selected set of        collectors 300.    -   Step 834: Obtain output associated with each collector 300 in        the selected set of collectors.    -   Step 870: Filter the output associated with each collector 300        to obtain filtered output associated with each collector 300.        Threshold the filtered output for each detector module 400        associated with a collector 300 in the selected set to obtain        thresholded filtered output associated with each collector 300.    -   Step 860: Combine the thresholded filtered output associated        with all of the collectors 300 in the selected set of collectors        to obtain combined thresholded filtered output.    -   Step 836: Analyze the combined thresholded filtered output.

The step of analyzing the combined thresholded filtered output may beperformed using any defect detection method, including those describedherein or any known defect detection method, such as those described inU.S. Ser. No. 10/864,962, entitled Method and System for ClassifyingDefects Occurring at a Surface of a Smooth Substrate Using GraphicalRepresentation of Multi-Collector Data, which is assigned to ADECorporation of Westwood, Mass. and which is herein incorporated byreference.

In the embodiment shown in FIG. 41, the method of obtaining combinedthresholded filtered output comprises obtaining combined channel chords550, which comprise the set of contiguous over-threshold filtered outputvalues in the output of an AOD scan for the set of selected collectors300. More specifically, in the embodiment shown in FIG. 41, the step 832of defining a channel 600 by identifying a selected set of collectors300 comprises identifying the dual back collectors 340A, 340B to form acombined back channel, and obtaining combined channel chords 550comprises obtaining combined back channel chords 540.

The method of obtaining combined channel chords 550 when it comprisesobtaining combined thresholded filtered output is shown in more detailin FIG. 42 and 43. It comprises the following:

-   -   The step 834 of obtaining output comprises obtaining output for        each AOD scan, further comprising obtaining the set of output        values from the AOD scan.    -   The output filtering portion of step 870 further comprises        obtaining filtered output for each AOD scan, further comprising        obtaining the set of filtered output values for the AOD scan.    -   The output thresholding portion of step 870 further comprises        thresholding the filtered output for each AOD scan, further        comprising obtaining the set of thresholded filtered output        values for the AOD scan, with thresholding comprising comparing        the output values V of the AOD scan elements 554 in a        collector's AOD scan line 532 against a threshold output value        V₀ and identifying which elements 554 are over threshold.    -   The step 860 of combining the thresholded filtered output        further comprises identifying channel chords 552 in the        thresholded filtered output, and, from them, forming combined        channel chords 550.    -   The step 836 of analyzing the combined thresholded filtered        output then further comprises a step 838 of analyzing the        combined channel chords 550.

As seen in FIG. 42A, in a selected set of collectors 300, each collector300A, 300B provides as output a selected number of AOD scan lines 532,forming a collector scan 534 and comprising a selected number of scanelements 554. A channel chord 552 comprises the set of contiguous AODscan elements 554 in a collector's AOD scan line 532 having outputvalues V greater than a threshold output value V₀. Output fromcollectors 300A, 300B may be used to form a combined channel chord 550.

The step 838 of forming combined channel chords 550 comprises overlayingthe output from an AOD scan 532 associated with the selected set ofcollectors 300 in the following manner: Each AOD scan line 532 in acollector's scan 534 has another AOD scan line 532 associated therewithin the scan 534 of the other collectors in the selected set, the scanlines 532 so associated forming a scan line set 536 such that all scanlines 532 in a collector's scan 534 are members of separate scan linesets 536. Further, each AOD scan element 554 in an AOD scan line 532 hasanother AOD scan element 554 associated therewith in each of the AODscan lines 532 in the m scan line set 536, the scan elements 554 soassociated forming a scan element set 538 such that all scan elements554 in an AOD scan line 532 are members of separate scan element sets538.

When output from a selected set of collectors 300 is combined, it iscombined on the scan element 554 level, with all of the scan elements554 in a scan element set 538 forming a combination element 556 thatrepresents the associated AOD scan elements 554 in a scan element set538.

A combined channel chord 550 comprises the set of combination elements556 for which at least one of the associated AOD scan elements 554 thatthe combination element 556 represents has an output value V greaterthan a threshold output value V₀. The magnitude of the combinationelement 556 in each combined channel chord 550 is the magnitude of oneof the AOD scan elements 554 represented thereby, preferably the AODscan element 554 having the greatest over-threshold output value V.

An example of the step 838 of forming combined channel chords 550 isshown in FIG. 42 b, in which the selected set of collectors 300 compriseback collectors 340A, 340B. While it is within the spirit of thisinvention for each collector to have associated therewith several AODscan lines 532, for the sake of this example, let there be only one AODscan line per collector. For example, the collectors 340A, 340B provide,respectively, AOD scan lines 532A, 532B. AOD scan line 532A has aplurality of scan elements, e.g. scan elements 554AA, 554AB. AOD scanline 532B also has a plurality of scan elements, as an example 554BA,554BB. Scan elements 554AA, 554 AB, would. be considered to haveassociated with them, respectively, scan elements 554BA, 554BB.Therefore, scan elements 554AA, 554BA would form a scan element set 538,and scan elements 554AB, 554BB would form a separate scan element set538.

In FIG. 42 b, the scan elements 554 that have an output value V greaterthan a threshold output value V₀ are shown as dark, forming a chord 552.The set of combination elements 556 for which at least one of theassociated AOD scan elements 554 that the combination element 556represents has an output value V greater than a-threshold output valueV₀ are also shown as dark, forming a combination chord 556.

FIG. 43 shows channel chords 552 from a set of five collectors, and theset of combined channel chords 550 formed therefrom. The formation ofcombined channel chords 550 results in the recording of synchronousevents therein.

By defining channels 600 out of selected combinations of collectors 300,individual collector filtered output can be consolidated into scatter“fields”, which can then be used to facilitate defect detection ofdefects such as scratches.

The method of defect detection in which output associated withcollectors is independently or individually processed is particularlyhelpful in identifying asymmetric defects. Further, scratch detection isfacilitated by independently or individually processed collector output.

Scratches, also known as line defects, are difficult to identify becausetheir scatter forms a very narrow geodesic on the scatter hemisphere.Since each collector is responsive to scatter in a different region ofthe scatter hemisphere, data related to a scratch will appear as outputin different collectors. In addition, cross-scan filtering attenuateslinear defect signatures. When output associated with multiplecollectors is combined, the portion of the combined output related toscratches will not produce a sufficient level of output signal to exceedthe thresholding value. Therefore, it is preferable to analyze theoutput associated with each collector for individual detection of linedefects. Once the output data are filtered and tested to determine ifthey exceed a threshold value, the data that exceed the threshold valuesmay be analyzed, alone or in combination with other data, usingcurrently known techniques to identify line defects.

In addition, when output associated with the P-polarized and S-polarizedwing collectors is individually processed, channels may be defined forthe separation of the wing response for enhanced scratch detectionsensitivity and improved detection and classification of additionaldefect types.

Using the methods described herein, one may define multiple channels 600out of a single collector 300 or a set of collectors 300. For example,in FIG. 44, wing collectors 310A, 310B are shown. The wing collectors310A, 310B may be operated in both P and S configurations, andtherefore, the output associated with them may be used to form a wing(P) channel 61 OP and wing (S) channel 610S

In addition, when combining collectors 300 to form channels 600, themethods described herein may be combined to generate multiple channels600 for use in different applications. For example, as shown in FIG. 45,the output associated with the back collector 340A and the backcollector 340B may be filtered and processed in the conventional manner(the FT method 813) to form, respectively, the back channel 640A and theback channel 640B. Alternatively, the output may be processed inaccordance with the combined scatter method (the CFT method 812;combining collector output, then filtering and then thresholding theoutput) to form back combined (CFT) channel 641, or it may be processedin accordance with the individual collector processing method (the FTCmethod 814; filtering and thresholding the individual collector output,then combining output) to form back combined (FTC) channel 642. Finally,the output may be processed in accordance with the dual/CFTC method 816(combining the combined scatter method and the individual collectorprocessing method) to form a back combined (dual) channel 643.

Returning to FIG. 46, a data reduction module 670 may be provided foreach of the desired combinations of collectors 300. In the presentlypreferred but merely illustrative embodiment described herein, thefiltering and thresholding step 870 would be performed on the data inthe data acquisition nodes 570 and the data reduction nodes 670 for boththe combined scatter method 812 and the individual collector processingmethod 814. The combining step 860 for the combined scatter method (CFTmethod 812) would be performed on the data in the data reduction nodes670. For the individual collector processing method (FTC method 814),the combining step 860 would be performed using software in the systemcontroller and processing unit 500.

Method for Haze Analysis in a Multi-Collector Surface Inspection System

Defect detection, measurement confidence, and understanding of processessuch as wafer production and manufacturing processes are improved whenthe contribution of surface roughness on a scattering workpiece surfaceis known and taken into account. A workpiece surface can be said to havean amplitude and a spatial frequency, with the spatial frequencyrepresenting the density of the elements on the surface that causescatter (such as roughness or defects), and the amplitude comprising theheight of the elements on a surface. A surface structure comprises theaggregate of the elements (such as roughness or defects) on or in theregion of a surface. A surface structure's roughness may be quantifiedin any conventional manner, one being the average distance of thesurface from the mean surface of the wafer. A surface structure'sspatial frequency is determined by the density of the elements of whichthe structure is comprised. FIGS. 91 a and 91 b are illustrations of aworkpiece surface structure, showing surface structures S1 and S2, eachhaving elements E that cause scatter, structure S1 having elements E11,E12, and so on, and structure S2 having elements E21, E22, and so on. Itcan be seen that, while the spatial frequencies of surface structure'sS1, S2 are essentially identical (the elements E of which they arecomprised having essentially the same density), the elements E onstructure S2 have greater amplitudes than the elements of structure S1,so structure S2 can be seen to have a higher roughness value thanstructure S1.

As an incident beam's photons impinge the elements of a surface, photonsfrom the incident beam scatter off of the elements. The photons, whichtravel at a frequency that is determined by the incident coherent beam,scatter at a rate that is determined by the roughness of the surfacestructure. The rougher the surface (the higher the roughness value), themore photons are scattered. The intensity of scatter is determined, forthe most part, in a defect free region, by amplitude of the surfacefrequency. Returning to FIGS. 91 a, 91 b, structure S1, which has ahigher amplitude than structure S2, can be seen to scatter more photonsthan structure S2.

The direction of the photon scatter (the angle at which the photonsscatter) is largely determined by the spatial frequency of the surfacestructure. As the surface structure's spatial frequency increases, thedensity of the elements of which the structure is comprised increases,and so the angle at which the photons scatter off of the elementsbecomes more acute relative to the incident beam, shifting back towardthe incident beam.

A surface structure's spatial frequency may be divided into componentscomprising surface structure spatial frequency ranges. For example, FIG.91 a shows a graph of a surface height profile of a model surfacestructure comprising a region of a surface S of a workpiece W. Usingcommonly known mathematical techniques such as a Fourier transform, thewaveform representative of the model surface structure may be expressedby waveform components. In the example, referring to FIG. 92 b, thewaveform representative of the model surface structure may be expressedby, specifically, a high surface structure spatial frequency waveformcomponent 261 having a spatial frequency in a high surface structurespatial frequency range, a medium surface structure spatial frequencywaveform component 262 having a spatial frequency in a medium surfacestructure spatial frequency range, and a low surface structure spatialfrequency waveform component 263 having a spatial frequency in a lowsurface structure spatial frequency range. For sake of illustration, themodel surface structure was selected so that the waveform defined by itssurface height profile could be expressed by waveform components 261,262, 263 with equivalent amplitudes.

As photons scatter from a surface structure, the angles at which theyscatter can be modeled using the waveform components of the waveformthat is representative of the surface structure. In addition, the extentof scatter that will be present in a region above the surface can bemodeled using the waveform components of the waveform that isrepresentative of the surface structure. For example, FIG. 92 a-92 d arediagrams that show regions representative of the amount of and directionof photons scattered from a surface structure, by waveform component.FIG. 93 a shows scatter from the model surface structure of FIG. 91 b,with scatter associated with high surface structure spatial frequencywaveform component 261 scattering into high surface structure spatialfrequency surface scatter region HFS, scatter associated with mediumsurface structure spatial frequency waveform component 262 scatteringinto medium surface structure spatial frequency surface scatter regionMFS, and scatter associated with low surface structure spatial frequencywaveform component 263 scattering into low surface structure spatialfrequency surface scatter region LFS.

As the amplitude of the surface structure changes, the relativecontributions of the spatial frequencies to the waveform representativeof the surface structure change, and the scatter pattern changescommensurate with the changes in the waveform and its components. FIG.92 b shows an increase in scatter in the low surface structure spatialfrequency surface scatter region LFS which would be occur if the lowsurface structure spatial frequency waveform component of the waveformrepresentative of the surface structure had an increase in amplituderelative to the other waveform components. FIG. 92 c shows an increasein scatter in the medium surface structure spatial frequency surfacescatter region MFS which would be due to an increase in amplitude in themedium surface structure spatial frequency waveform component relativeto the other waveform components. FIG. 92 d shows an increase in scatterin the high surface structure spatial frequency surface scatter regionHFS due to an increase in amplitude in the high surface structurespatial frequency waveform component relative to the other waveformcomponents.

As noted above, in a multi-collector surface inspection system, such assystem 10, collectors are positioned at selected positions in the spaceabove a workpiece, with each collector responding to a specific range ofsurface structure spatial frequencies. FIG. 94 is a diagram showing thepattern of surface scatter observable by collectors in a system 10 inthe space above the surface of a workpiece and representative surfacestructure spatial frequency ranges associated therewith. It can be seenthat scatter associated with the low surface structure spatial frequencywaveform component 263 is observable by the front collector 330 and wingcollectors 310A, 310B, while scatter associated with the medium surfacestructure spatial frequency waveform component 262 is observable by thecenter collector 320 and wing collectors 310A, 310B, and scatterassociated with the high surface structure spatial frequency waveformcomponent 261 is observable by the back collector 340A, 340B. Therefore,it can be seen that surface structure spatial frequency data associatedwith scatter from a surface structure is obtainable from multi-collectorsurface inspection system, such as system 10.

Determining Surface Roughness of the Workpiece Surface Using theProportionality of Scatter Power Over a Range of Spatial Frequencies

It is preferable to minimize or eliminate the contribution of surfaceroughness from discrete defects from surface contamination. Therefore,it is preferable to identify the extent of the contribution of surfaceroughness in order that the extent of the contribution of surfaceroughness may be subtracted from the output associated with thecollector-detector assembly 200.

In accordance with another aspect of this invention, in a surfaceinspection system having a plurality of collectors, each of which isdisposed at a selected collection solid angle, comprising a selectedsolid angle above a scattering surface, a method for determining anextent of a contribution of surface roughness on the scattering surfacecomprises determining an extent of a contribution of surface roughnessfrequencies on the scattering surface. One aspect of the inventionfurther comprises monitoring surface structure spatial frequencycontributions to collector signal. In a further aspect, the methodcomprises monitoring surface structure spatial frequency contributionsto the workpiece surface using data from a set of collection solidangles in the space above the workpiece. In another aspect of theinvention, the method comprises determining an extent of a contributionof surface roughness frequencies on the scattering surface at a set ofcollection solid angles that are associated with a selected set ofcollectors.

In another aspect of the invention, the method comprises collecting“low-surface structure spatial frequency” variations of scatter at oneor more selected collection solid angles, whereby the amplitude of thescatter over the selected collection solid angle is proportional to theamplitude of surface variation causing the scatter, over a range ofsurface structure spatial frequencies detected at the selectedcollection solid angles.

In a still further aspect of this invention, the contribution/presenceof surface roughness frequencies on a scattering surface is determinedby displaying a histogram showing the amplitude of scattered photons (inparts per million/billion) for the selected solid angles.

Providing surface amplitude information for specific spatial frequenciesso as to determine an extent of a contribution of surface roughness onthe scattering surface (conducting haze analysis) allows correlations tobe developed between scatter intensity values² and wafer features suchas the extent of “grain” or, as referred to in the Stover reference,“surface lay” of the silicon surface or the extent or type of surfacestructures. Such correlations will then allow insight into the outcomeof processing the wafer surface (such as to test the results of chemicalmechanical planarization (CMP). In a further aspect of the invention,the step of providing surface amplitude information for specific spatialfrequencies further comprises combining output associated with a set ofselected collectors to form a haze field, comprising haze associatedwith combinations of collectors outside of the plane of incidence (e.g.backs and wings). Forming haze fields from output associated with a setof selected collectors is useful in minimizing the effects of incidentbeam orientation to the “grain” of the silicon surface. By collectingsymmetrically above the wafer, a system 10 is able to reduce theintensity variations caused by the orientation of the incident to thesurface “grain”. One such set of selected collectors, the combination ofoutput associated with which has shown to be useful in minimizing theeffects of incident beam orientation to the “grain” of the siliconsurface, comprises the back collectors.

The idea reflects the proportionality, not absolute determination, of ssurface structure spatial frequency and direction of scatter. The methodis based on the idea that each collector responds to scattered lightassociated with a specific range of surface structure spatialfrequencies. The theory indicates the “best-case” response range foreach collector. Since the response range is constant within a givenmeasurement configuration, e.g. incident beam angle, wavelength,collector dimensions, etc., valid relationships may be drawn betweensurface structure and the direction of surface roughness scatter.

It should be apparent to those of skill in the art from thisillustration that the present invention is not limited to the particularalgorithm described herein, and that other approaches and other specificalgorithms may be used to process the data obtained from the variousdetector modules 400 and to determine defect geometry and classifydefects in accordance therewith.

In accordance with the present invention, and as shown in FIG. 75, amethod 970 for determining an extent of a contribution of surfaceroughness frequencies on the scattering surface comprises the followingsteps:

-   -   Step 971: Determine the in-scan surface structure spatial        frequency response 702 of a collector 300 to scatter light,        comprising light that has been scattered from a surface S by an        incident beam that is applied at a selected angle from normal        onto the surface.    -   Step 972: Determine a response range comprising the range of the        in-scan surface structure spatial frequency response 702.    -   Step 973: Determine a scatter intensity value representative of        the scattered light for the response range in order to determine        an extent of a contribution of surface roughness on the        scattering surface.

In the presently preferred yet merely illustrative embodiment of thepresent invention, the incident beam angle comprises about 65 degreesfrom normal.

The method of the present invention further comprises

-   -   Step 974: Determine a scatter intensity value for each collector        300 in a set of selected collectors 300, and compare scatter        intensity values in order to build an understanding of the haze        response by the surface to impingement of an incident beam        thereon.

The method of the present invention further comprises

-   -   Step 975: Determine a scatter intensity value for a collector        for a plurality of surfaces, and compare scatter intensity        values in order to build an understanding of the haze response        by the plurality of surfaces to impingement of a coherent beam        thereon.

In a further aspect of the present invention, the scatter intensityvalue comprises a value that is an amplitude of the scattered light. Inthe presently preferred yet merely illustrative embodiment of thepresent invention, the scatter value comprises a value representative ofa maximum amplitude of the scatter light. Alternatively, the scattervalue comprises a value representative of a minimum amplitude, a valuerepresentative of the difference between a minimum and maximum value, ora value derived from any desired function of the scatter light.

In the presently preferred yet merely illustrative embodiment of thepresent invention, the step 971 of determining the in-scan surfacestructure spatial frequency response of each collector further comprisescreating a surface structure spatial frequency plot 705 in which thecross-scan surface structure spatial frequency 704 for each collector ismapped against in-scan surface structure spatial frequency 702, formingthe spatial frequency response region 700 for each collector 300 in themulti-collector surface inspection system 10. One such surface structurespatial frequency plot is shown in FIG. 52, which presents the surfacestructure spatial frequency response defined by the cross-scan surfacestructure spatial frequency response 704 and the in-scan surfacestructure spatial frequency response 702 for a set of collectors 300comprising the front collector 330, center collector 320, dual wingcollectors 310A, 310B, and back collectors 340A, 340B of the presentlypreferred yet merely illustrative embodiment of the present invention.The mapping results in a visual representation of a spatial frequencyresponse region 700 for each collector 300, such as front collectorspatial frequency response region 730, a center collector spatialfrequency response region 720, dual wing collectors spatial frequencyresponse regions 710A, 710B, and back collector spatial frequencyresponse regions 740A, 740B.

As seen in FIG. 52, each collector 300 responds over a specific range ofwafer surface structure spatial frequencies. The response range isconstant within a given measurement configuration, e.g. incident beamangle, wavelength, collector dimensions, etc. Mapping surface structurespatial frequency response in terms of cross-scan surface structurespatial frequency response 704 and the in-scan surface structure spatialfrequency response 702 produces a means of monitoring the varioussurface structure spatial frequency contributions to surface roughnessscatter.

It can also be seen that the origin in the spatial frequency plot 705 isoffset to reflect the offset of the incident beam angle from surfacenormal of the sample. The step of determining response ranges furthercomprises. using the surface structure spatial frequency plot toidentify ideal response ranges 760 for each collector 300.

In the presently preferred yet merely illustrative embodiment of thepresent invention, the step 974 of comparing the scatter intensityvalues further comprises a step 976 of displaying the scatter intensityvalues for each response in a visual representation. The displaying step976 may comprise forming a chart 706, as shown in FIG. 54, identifyingthe scatter intensity values. Alternatively, as shown in FIG. 53, thedisplaying step 976 may comprise a step 977 of mapping the idealresponse ranges 760 into a histogram 780 illustrating the scatter value.

The histogram 780 of FIG. 53 has an element 782 associated with eachideal response range 760, with the height of an element 782 comprising apower value representative of the power scattered at each collector 300within the response range 760. In a presently preferred yet merelyillustrative embodiment of the present invention, the power valuecomprises a value representative of an amount of scatter measured (forexample, in parts per million, or nanowatt per steradian, or ppm persteradian).

In a still further embodiment, the step 977 of mapping the idealresponse ranges 760 into a histogram 780 further comprises having the.histogram 780 illustrate the breadth of the ideal response range 760associated with each collector 300. As shown in FIG. 53, each element782 on the histogram 780 is provided with a width that is representativeof the ideal response range 760 associated therewith.

In a still further embodiment, as shown in FIG. 53, the histogram 780illustrates the surface structure spatial frequency values of thein-scan spatial frequency responses 702 associated with the scatteredlight by placing the elements at locations on the histograms 780 thatrepresent the range of the in-scan spatial frequency responses 702. Forexample, the back collector spatial frequency response regions 740Ademonstrates a range of responses between in-scan spatial frequency740A₁ and in-scan spatial frequency 740A₂. The difference between thefrequencies 740A₁, 740A₂ determines the breadth of the histogram element782A associated with the back collector spatial frequency responseregions 740A. Referring to FIG. 53, it can be seen that certain of thesurface structure spatial frequency responses overlap. For example, thehistogram element 820 associated with the center collector surfacestructure spatial frequency response region 720 overlaps with thehistogram element 840A (associated with the back collector surfacestructure spatial frequency response region 740A), the histogram element810A (associated with the wing collector surface structure spatialfrequency response region 710A), and the histogram element 830(associated with the front collector surface structure spatial frequencyresponse region 730. Therefore, when a histogram 780 is created for thesurface structure spatial frequency responses 760, as seen in FIG. 53,the elements 782 associated with the overlapping surface structurespatial frequency response 760 also overlap on the histogram 780.

Overlapping data may be displayed in any conventional manner, such ascolor changes, hash marks, or using overlapping transparencies. Inaddition, a decision could be made to ignore data having certaincharacteristics or associated with certain collectors. For example, dataassociated with the wing collectors operating in the P configuration maybe excluded from the analysis in order to reduce the extent of overlap.Such exclusion would provide minimal impact on the analysis, since mostsurface scatter would be filtered from the wing data due topolarization.

The histogram 780 from FIG. 53 may also be flipped horizontally topresent the histogram 784 in FIG. 55, in which the x-axis is oriented ina more common manner, showing low to high frequency.

As noted above, the method of the present invention further comprisesdetermining a scatter intensity value for a collector for a plurality ofsurfaces by impinging an incident beam on the plurality of surfaces, andcomparing scatter intensity values in order to build an understanding ofthe haze response by the plurality of surfaces. Scatter intensity valuesand surface structure spatial frequency response ranges for a pluralityof surfaces may thus be used to identify and highlight differences insurface roughness that are due to differences in processing, such aspressure polish time, slurry type, chemical concentrations. The chart ofFIG. 54 identifies scatter intensity values for three different waferprocesses. Levels of surface structure spatial frequencies (such as low,medium and high) may be assigned different colors so that thesignificant differences in the haze in wafer created using the differentprocesses are readily shown.

The scatter intensity values associated with surface roughness³ may benormalized for each collection area represented in a histogram such ashistogram 784 in order to produce a low-resolution power spectraldensity (PSD)-type chart. The power spectral density (PSD) of aquasi-stationary random process is the Fourier Transform of theautocovariance function. Specifically, the spectral density Φ(ω) of asignal f(t) is the square of the magnitude of the continuous Fouriertransform of the signal.

${\Phi(\omega)} = {{{\frac{1}{\sqrt{2\pi}}{\int_{- \infty}^{\infty}{{f(t)}{\mathbb{e}}^{{- {\mathbb{i}\omega}}\; t}{\mathbb{d}t}}}}}^{2} = {{F(\omega)}{F^{*}(\omega)}}}$

where ω is the angular frequency (2π times the cyclic frequency) andF(ω) is the continuous Fourier transform of f(t).

Power spectral density is usually expressed in units squared perfrequency. For two-dimensional roughness, this would beum²/(cyles/um)²—quivalent to um⁴. The contribution of surface roughnesson a scattering surface to scattered light may be calculated from amountof scattered light observed in the solid angle associated with eachselected collector. FIG. 76 is a diagram illustrating the solid angle ofcollection of scatter signal from a surface S at a collector 300. FIG.76 shows an incident beam A_(i) with an incident power P_(·i) and λ=532m, impinging on a surface S, producing a reflected beam A_(r) having areflected power P_(r)Ps for the solid angle defined by an angle θ_(c)that is disposed at a scattering angle θ_(s)ω is: 2π times the cyclicfrequency). Therefore, ω may be determined to be:Ω=2π(1−cos θ_(c)),

where θ_(s) is the scattering angle.

The Bidirectional Reflectance Distribution Function BRDF is thedifferential ratio of the sample radiance normalized by its irradiance.Therefore, once the ω is determined, one can then determine

${BRDF} \cong \frac{P_{s}/\Omega}{P_{i}\cos\;\theta_{s}}$

where

-   -   P_(i)=Incident power, and    -   P_(s)=Scattered power.

When all scattered light is being collected across the area of thehemisphere, and the total power measured at each collector is summed,the RMS roughness σ_(rms) may then be determined by

${\sigma_{rms} \cong {\frac{\lambda}{4\pi}\sqrt{\frac{P_{s}}{P_{r}}}}};$

It is preferable to minimize or eliminate the contribution to scattersignal of surface roughness so that discrete defects can bedistinguished from surface contamination. Therefore, it is preferable toidentify the extent of the contribution of surface roughness in orderthat it may be subtracted from the output associated with thecollector-detector assembly 200. Further, it is preferable to track hazefor each of the collectors in order to understand the confidence in themeasurement. The composition of all collectors is used to providesurface roughness information. RMS Roughness is derived from summing allof the scatter intensity values for each collector. Further detail onRMS roughness—based haze analysis is found below.

It is further preferable to identify the extent of the contribution ofsurface roughness to collector output in order to analyze collectorresponse to light scattered from surface structural conditions.

Visualization of Spatial Frequency Distributions Occurring in aWorkpiece Surface Structure

Typically, surface inspection tools that measure RMS roughness have anormal incident beam. The surface inspection system 10 differs fromsurface inspection tools that measure RMS roughness in that it has anoblique incident beam and it collects scatter from collectors disposedat selected angles. The present invention involves an improvement inhaze analysis comprising analyzing the spatial frequency distributionsin surface scatter. As noted above, haze analysis, or analysis of thediminished atmospheric visibility that results, in the case of a surfaceinspection tool, from light scattered from a surface, is typicallyperformed in order to analyze collector response to light scattered fromsurface structural conditions. Examples of systems and methods thatprovide haze analysis include those of the '701 patent, as well as U.S.Pat. Nos. 6,118,525 and 6,292,259, all of which are assigned to ADEOptical Systems Corporation and all of which are herein incorporated byreference.

In a multi-collector surface inspection system such as system 10described above, the summation of haze from each of the plurality ofsurface collectors 300 is approximately the haze observed by a totalintegrated scatter tool, such as is described in the Stover reference.The placement of the plurality of collectors (front, center, backs, andwings) in the surface inspection system 10 described above, at theirrespective locations allows for the collection of angular information onlight scattered from a surface, which, as noted above, can facilitateseparation of scattered light into the spatial frequency ranges ofsurface structures, which then in turn can be used to provide detailabout the kind of structures causing the surface roughness.

In addition, scattered light having selected light characteristics, suchas a selected polarization, may be used to identify the source of thescatter on or in a workpiece surface. For example, scattered light of aselected polarization may be used to distinguish between surfaceroughness and a surface defect. When a P-polarized incident coherentbeam hits a workpiece surface, it is rotated 90 degrees, and scatterfrom surface roughness becomes S-polarized at the wing locations.However, scatter caused by a P-polarized incident coherent beam hittinga particle on a workpiece surface remains P-polarized at some ratio,especially in scatter observable in the solid angles occupied by thewing collectors. Therefore, polarization may be used to distinguishbetween particles and surface roughness, especially in the wingcollectors that were located, as described above, in a location in whichP-contribution from scatter associated with surface roughness is at aminimum. In addition, power measured at each collector can be summedwith all other collectors to produce total (measured) scatter power(TIS).

Therefore, it is an object of the present invention to use knowledge ofthe characteristics of light scattered from surface structuralconditions in order to improve the analysis of collector response tolight scattered from surface structural conditions.

It is a further object of the present invention to use characteristicsof the light scattered from surface structural conditions, such asintensity of the scattered light and surface structure spatial frequencyrange associated with the scattered light, to identify characteristicsof the surface structural conditions, such as average surface roughnessover a selected range of wafer surface structure spatial frequencies.

It is a further object of the present invention to monitor workpieceproduction processes using characteristics of scattered light fromworkpiece surfaces, comprising intensity of the scattered light andsurface structure spatial frequency range of the scattered light.

With the present invention, there is provided a method and a system forinspection of surface structural conditions of a workpiece, over a rangeof surface structure spatial frequencies determined by system geometry,by analyzing surface scatter, comprising light scattered from a surfaceand comprising surface roughness over a determinable surface structurespatial frequency range, involving analyzing relationships betweenportions of the scattered light associated with defined surfacestructure spatial frequency ranges. In one aspect of the invention,analyzing surface scatter involves treating the power measured in thecollectors from the surface scatter, over a determinable range ofsurface structure spatial frequency responses, as a variable in theanalysis.

In one embodiment of the invention, the method comprises separatingsurface roughness scatter into surface structure spatial frequencyranges, where direction of surface roughness scatter is predominatelydetermined by incident beam properties and the idealized spatialfrequency of the structure scattering the light, and analyzing at leasta first scatter portion comprising a portion of the surface roughnessscatter within a first surface structure spatial frequency range. In afurther aspect, the method comprises analyzing the first scatter portionalone or in combination with a second scatter portion comprising aportion of the surface roughness scatter within a second surfacestructure spatial frequency range. In a still further embodiment, themethod comprises analyzing a plurality of scatter portions of surfacescatter, each scatter portion comprising a portion of the surfaceroughness scatter having surface structure spatial frequencies within aselected spatial frequency range defined by the architecture of thesurface inspection system.

The method comprises observing the presence of surface roughness scatterwithin an area above the surface, with the area being selected for thepower range of the power measured over the measurable spatial frequencywithin the area, in order to observe surface roughness scatter byspatial frequency over the selected frequency range, and to analyzesurface roughness scatter by power measured over a known area.

In a further aspect, the method comprises collecting the surface scatterin a plurality of areas above the surface, with each area being selectedfor its association with a selected spatial frequency range in thefrequency of surface roughness in a surface structure, order to analyzethe surface roughness of the surface structure by the selected spatialfrequency ranges and to analyze relationships between surface scatterassociated with different ones of the spatial frequency ranges.

In one embodiment, collecting the surface roughness scatter at aplurality of areas comprises positioning a plurality of scattered lightcollectors at selected positions above the surface, with each positionselected so that the scattered light collector, at the position, is ableto observe power of a surface roughness scatter associated with adeterminable surface structure spatial frequency range. The methodfurther comprises observing surface roughness scatter at the scatteredlight collectors and analyzing the surface roughness scatter byscattered light collector. In a further embodiment, the step ofobserving surface roughness scatter comprises identifying the presenceof surface scatter at the scattered light collectors and measuring anextent of the surface roughness scatter.

In another embodiment, the method further comprises using a plurality ofcollectors disposed at positions above the surface, identifying asurface structure spatial frequency range to be associated withscattered light observable by each of the collectors, and analyzing thesurface roughness scatter by scattered light collector.

In another aspect of the invention, analyzing surface scatter involvesdeveloping visual representations of haze produced by the surfaceroughness scatter, with haze comprising an atmospheric condition abovethe surface of diminished visibility that results from conditions suchas background noise or surface roughness, the visual representationsshowing the presence of haze arising from surface scatter, comprisingscattered light from an incident beam impinging on selected locations onthe surface, according to the spatial frequency range of the surfaceroughness scatter. In a further embodiment, developing visualrepresentations further comprises presenting an extent of the haze.

In a still further embodiment, developing visual representations furthercomprises presenting haze associated with surface roughness scatterassociated with a plurality of spatial frequency ranges, with the hazedisplayed according to the spatial frequency range with which itsassociated surface roughness scatter is associated. In furtherembodiments, presenting haze associated with surface roughness scatterassociated with a plurality of spatial frequency ranges furthercomprises identifying an extent of haze for each spatial frequencyrange.

In another embodiment, developing visual representations of hazecomprises developing composite haze maps, in which map positions areassociated with locations on the region under investigation, and whichshows multiple representations of haze associated with surface roughnessscatter arising from an incident beam reflected from each of saidlocations, with each representation of haze associated with surfaceroughness scatter associated with a different spatial frequency range.

According to another aspect of the present invention, there is provideda method for inspection of surface structural conditions of a workpieceby analyzing collector response to light scattered from surfacestructural conditions, with the system and method involving analyzing aportion of the light associated the a selected surface structure spatialfrequency range.

In a further aspect of this invention, a method and a system forinspection of surface structural conditions of a workpiece involvesobserving scattered light with a plurality of scattered lightcollectors, each collector disposed to observe scattered light over adeterminable surface structure spatial frequency range, said scatteredlight having intensity representative of surface roughness scatterhaving a determinable surface structure spatial frequency range,combining output associated with at least two of the selected scatteredlight collectors to form combined surface roughness scatter output, andanalyzing the spatial frequency contributions of the combined surfaceroughness scatter output.

In an even further aspect of the invention, analyzing the spatialfrequency distributions further comprises forming visual displays ofsurface roughness scatter output. In a further embodiment, formingvisual displays comprises developing displays in which spatial frequencydistributions are represented by a display element, with each spatialfrequency to be presented in the display being associated with a displayelement. Preferably, the display element comprises a display color. In afurther embodiment, forming visual displays comprises developingdisplays showing an extent of the surface roughness scatter output of aselected spatial frequency, in order to identify the relativecontribution of light associated with the selected spatial frequency inthe light of the surface roughness scatter output.

In addition, forming visual displays comprises constructing compositehaze maps, further comprising developing workpiece surface maps in whichmap regions are associated with surface structures on the workpiece, andin which a characteristic of the display, such as graphical elements orcolor, in a map region identifies the relative contribution of roughnesshaving a selected spatial frequency in the surface structure associatedwith the map region. A further embodiment comprises defining channels ina surface inspection system, and constructing composite haze mapsfurther comprises representing surface roughness scatter outputassociated with selected defined channels in a haze map.

In an even further aspect of the invention, analyzing the spatialfrequency distributions further comprises forming graphicalrepresentations of the spatial frequency distributions. In a furtherembodiment, developing graphical displays further comprises developingbar charts of the measured power by the spatial frequency responserange.

When light is scattered from surface structural conditions and observedby a collector positioned above the surface, the intensity of theportion of the light that is present in the space defined by the solidangle of the collector, coupled with the identification of the spatialfrequency range of the surface structure from which the portion isscattered, allows analysis of the portion by frequency range.Determining an extent of a contribution of surface roughness on thescattering surface allows correlations to be developed between scatterintensity values and wafer features such as the extent of “grain” of thesilicon surface or the extent or type of surface structures.

One aspect of the invention further comprises monitoring spatialfrequency contributions to surface roughness scatter. In a furtheraspect, the method comprises monitoring spatial frequency contributionsto surface roughness scatter at a set of collection solid angles that isassociated with a selected set of collectors.

Since the response range is constant within a given measurementconfiguration, e.g. incident beam angle, wavelength, collectordimensions, etc., valid relationships may be drawn between surfacestructure and the direction of surface roughness scatter. Scatterintensity values may be compared in order to build an understanding ofthe surface response by haze levels.

In a surface inspection system such as system 10, in which a pluralityof collectors are disposed above a surface, the identification ofscattered light's intensity and frequency range allows sorting of lightthat is scattered from surface structural conditions by surfacestructure spatial frequency range and the use of surface structurespatial frequency range as a variable in the analysis of scatteredlight. Haze associated with a selected surface structure spatialfrequency range may then be analyzed alone or in combination with hazeassociated with other selected surface structure spatial frequencyranges.

Collectors such as the collectors 300 in the surface inspection system10 may be used to measure the intensity of light scattered from surfacestructural conditions, and positioning the collector in the space abovethe workpiece relative to the angle of the incident beam impinging onthe surface may be used to associate the collector output with aselected surface structure spatial frequency range. It is within thescope of the present invention to place collectors at selected positionsin the space above a surface under investigation, with the positions soselected to optimize the presence of haze associated with surfacescatter propagating from a surface having a specific surface structurespatial frequency.

It should be noted that the output obtained by a collector such ascollector 300 in a collection and detection assembly 200 does notidentify the presence of scatter having a particular frequency. Acollector's output indicates the presence of light in the space that isdefined by the solid angle of the collector. When the light results fromincidence of a coherent beam from a workpiece surface, it is theposition of the collector in the space above the surface that defines toa large extent the observable scattered light for the collector. Theobservable scattered light will be that portion of the scattered lightthat is associated with the range of spatial frequency of the surfacestructure impinged upon by the incident beam. t. Therefore, it is theknowledge of the spatial frequency range of surface structure that isassociated with the scattered light that is observable by a collector(obtained from knowledge of the position of the collector relative tothe surface) that allows the observation of surface scatter associatedwith a particular spatial frequency.

In addition, it should be noted that the output obtained by a collectorsuch as collector 300 in a collection and detection assembly 200indicates the amplitude of the roughness of the structures in and on theworkpiece surface by identifying the intensity of light scattered fromthe workpiece surface when a coherent beam is incident from the surface.Output from collector 300 comprises voltage signals that are indicativeof photon activity within a collector, with the photon activityresulting from light scattered from the surface of the region underinspection, and with the extent of the voltage signal being indicativeof the extent of the intensity of such photon activity. In addition, theextent of the intensity of such photon activity at the collectorindicates the amplitude of the roughness of the structures in and on theworkpiece surface.

The intensity of the light scatter indicates the amplitude of theroughness of surface structures because, first, the scatter's intensityat the collector is proportional to the amplitude of the light wavescomprising the scatter, and, second, the amplitude of the scatteredlight waves is proportional to the amplitude of the roughness of thestructures in and on the workpiece surface. The higher the roughness ofsurface structures, the higher the number of photons scattered from thesurface, and, in turn, the higher the number of photons collected, forexample in parts per million (ppm), by the collector. The output of acollector, being a voltage value representative of the number of photonsobserved in the space above a workpiece surface within a solid angleabout the collector, thus identifies the amplitude of structures in andon the workpiece surface.

Providing surface amplitude information for specific spatial frequenciesfurther comprises combining output associated with a set of selectedcollectors to form a haze field. Forming haze fields from outputassociated with a set of selected collectors is useful in minimizing theeffects of incident beam orientation to the “grain” of the siliconsurface.

It is known that scatter patterns, also known as haze patterns, differgiven the location above the workpiece surface at which the haze isobserved. For example, collectors located in the front quartersphere FQ(referring to FIG. 6, FQ being the region lying above the workpiecesurface, between the base plane B and the normal plane NP, through whichpasses the incident beam before it reaches the base plane B) receivescatter having more lower frequency components than do collectorslocated in the back quartersphere BQ or than do collectors in regionsalong or containing the normal plane NP. Therefore, front collectorswill register increased levels of lower spatial frequencies.

It is also known that scatter patterns differ given characteristics ofthe wafer under examination. For example, wafers that have beenprocessed with treatments (such as annealing or epitaxial processes)that result in smoother surfaces, tend to produce scatter with morelower frequency components than do wafers with polished surfaces. Giventhat front collectors will register scatter levels of lower spatialfrequencies more than will collectors in other locations above thesurface, light from the surfaces of annealed or epitaxial wafers willscatter more to front collectors than to collectors in other locations.

The following is an illustrative but not necessarily preferred methodfor analyzing surface scatter using a multi-collector surface inspectionsystem such as system lo to inspect workpieces such as wafers: Themethod, shown in FIG. 98, comprises a step 264 in which outputrepresentative of surface scatter is separated by surface structurespatial frequency associated with the surface scatter, and a step 268 inwhich the surface scatter is then analyzed by its associated surfacestructure spatial frequency range.

The step 264 of separating the output representative of surface scatterby the spatial frequency of the surface structures further comprises thefollowing steps:

-   -   In a step 265, a set of expected spatial frequency ranges is        selected for the surface structure to be observed. For example,        the selected frequency ranges could comprise a high surface        structure spatial frequency range, a medium surface structure        spatial frequency range and a low surface structure spatial        frequency range.    -   Step 266: Recognizing that collector placement above a test        surface determines the surface structure spatial frequency range        associated with the scattered light observable by the collector,        the collectors in a multi-collector surface inspection system 10        that will provide output associated with the selected surface        structure spatial frequency ranges are identified. For example,        a front collector 330 would be selected as the low surface        structure spatial frequency range collector for its ability to        observe scatter associated with surface structure having a        spatial frequency within a low surface structure spatial        frequency range, a center collector 320 would be selected as the        medium surface structure spatial frequency range collector for        its ability to observe scatter associated with surface structure        having a spatial frequency within a medium surface structure        spatial frequency range, and a back collector 340A, 340B, alone        or in combination as a channel 640, would be selected as the        high surface structure spatial frequency range collector for its        ability to observe scatter associated with surface structure        having a spatial frequency within a high surface structure        spatial frequency range.    -   Step 267: Surface scatter output is obtained for each of the        selected collectors in order to obtain output to be associated        with each selected surface structure spatial frequency range. If        desired, output of selected collectors is combined to create        output to be associated with a channel. For, example, in order        to obtain scatter surface output to be associated with a high        surface structure spatial frequency range, output from the back        collectors 340A, 340B could be combined using the methods        described above to obtain output associated with back combined        (CFT) channel 641, back combined (FTC) channel 642, back        combined (dual) channel 643, or another desired channel.

In one embodiment, as shown in FIG. 99, the step 268 in which thesurface scatter is then analyzed by its associated surface structurespatial frequency range may comprise the step 269 of analyzing theoutput associated with a portion of surface scatter alone or incombination with output associated with other portions of surfacescatter, with each scatter portion comprising a portion of the scatterfrom surface structure having a spatial frequency within a selectedsurface structure spatial frequency range. In one embodiment, the outputanalyzing step 269 comprises analyzing output associated with surfacescatter by scattered light collector.

The output analyzing step 269 is facilitated by a step 274 ofestablishing a visual representation to be associated with the outputassociated with each surface structure spatial frequency range in theanalysis. Establishing a visual representation further comprises thefollowing steps:

-   -   Step 275: A characteristic of the visual representation is        assigned to represent an identification of the presence of        surface scatter in the output. In one embodiment, the visual        representations are haze maps, with haze comprising an        atmospheric condition above the surface of diminished visibility        that results from conditions such as background noise or surface        roughness. Haze maps comprise maps of the surface of a wafer, in        which the positions on the map represent locations on a surface        that caused an observation of haze by a collector during the        reflection of an incident beam from the surface at the location.        The haze map has a characteristic assigned thereto to represent        the observation of surface scatter at a position on the haze map        that represents the location on the wafer surface at which haze        was observed.        -   FIG. 78 a, FIG. 78 b, and FIG. 78 c are examples of haze            maps for displaying haze associated with a set of frequency            ranges. FIGS. 78A, 78B, and 78C show wafer haze maps 271,            272, 273 for output associated with haze from scatter from            surface structures having a surface structure spatial            frequency within, respectively, the high surface structure            spatial frequency range, the medium surface structure            spatial frequency range, and the low surface structure            spatial frequency range. In FIG. 78A, 78B, and 78C, the            characteristic of the visual representation that represents            the observation of surface scatter (i.e., identifying the            presence of haze) is a graphical element, with a different            graphical element associated with each selected surface            structure spatial frequency range.        -   In FIG. 78A, the graphical element for identifying haze            associated with high surface structure spatial frequency            range comprises dots. In FIG. 78B, the graphical element for            identifying haze associated with a medium surface structure            spatial frequency-range comprises parallel lines in a first            direction. In FIG. 78C, the graphical element for            identifying haze associated with a low surface structure            spatial frequency range comprises parallel stripes of a            second direction. In another embodiment of the present            invention, the graphical representation could be color. For            convenience, the haze map colors may be chosen to be            consistent with the human visual spectrum, with blue            representing high surface structure spatial frequency            ranges, green representing medium surface structure spatial            frequency ranges, and red representing low surface structure            spatial frequency ranges. For purposes of illustrating this            embodiment of the present invention employing color, the            dots of FIG. 78 a could represent blue, the parallel lines            in a first direction of FIG. 78 b could represent green, and            the parallel lines in a second direction of FIG. 78 c could            represent red.    -   Step 276: An extent of surface scatter observed by each        collector is represented by variation in the visual        representation. In a further embodiment, the step 276 of        representing extent of surface scatter further comprises        presenting an extent of the haze. For example, presenting an        extent of the haze could comprise modifying the characteristic        to represent an extent of surface scatter.        -   In FIGS. 78A, 78B, and 78C, variation in the amount of haze            is shown by variation in the graphical element. In FIG. 78A,            variation in a high structure spatial frequency range is            shown by variation in dot density. In FIG. 78B, variation in            the amount of haze of a medium surface structure spatial            frequency range is shown by variation in density of the            parallel lines in a first direction. In FIG. 78C, variation            in the amount of haze in a low surface structure spatial            frequency range is shown by variation in density of the            parallel stripes in a second direction.        -   In the embodiment in which color represents surface            structure spatial frequency range, the variation in the            visual representation could be shown by variation in the            intensity of the color associated with the surface structure            spatial frequency range, with no scatter represented by no            color, a low amount of scatter represented by color of low            intensity, and the maximum amount of scatter represented by            the most intense color. For purposes of illustrating this            embodiment of the present invention employing color, the            density of the dots of FIG. 78A could represent the            intensity of blue, the density of the parallel lines in a            first direction of FIG. 78B could represent the intensity of            green, and the density of the parallel lines in a second            direction of FIG. 78C could represent the intensity of red.        -   The step 276 of representing an extent of surface scatter by            variation in the visual representation could comprise a step            277, in which the values representative of the extent of            haze are mapped into values for the extent of variation in            the visual representation. In the embodiment in which            graphical elements are modified to represent scatter            intensity, for each graphical element, each scatter            intensity value could be mapped into a value representative            of the amount of density of the graphical element. In the            embodiment in which color intensity is used to represent            scatter intensity, for each color, each scatter intensity            value could be mapped into a pixel color value. For example,            if the display system provides 256 levels of a color, each            scatter intensity value could be mapped into a pixel color            value ranging from 0 to 255. The manner in which the values            representative of the extent of haze is assigned to values            for the extent of variation in the visual representation is            described in more detail below.    -   Step 280: For a set of surface scatter output associated with a        defined surface structure spatial frequency, the visual        representations are constructed, using the assigned        characteristic of the visual representation to represent an        identification of the presence of surface scatter in the output,        the assigned variation in the visual representation to represent        an extent of the surface scatter, and values for the extent of        variation in the visual representation to represent the values        representative of the extent of haze.        -   In the embodiments described above, the resultant visual            representations will comprise haze maps, in which each pixel            in the display is associated with a location on the surface            under investigation, and in which each pixel displays a            variation in visual representation representative of an            amount of haze observed by a collector during the incidence            of a coherent beam on the surface at the location associated            with the pixel.        -   The step 280 of constructing visual representations could            comprise a step 284 a of constructing one haze map for each            selected surface spatial frequency range, or it could            comprise a step 284 b, comprising constructing a composite            haze map by combining maps for at least two selected surface            spatial frequency ranges into a single map. FIG. 79 shows an            example of a composite haze map 306, created by            superimposing the haze maps 271, 272, 273 shown in FIGS.            78A, 78B, and 78C.

The step 277, in which the values representative of the extent of hazeare mapped into values for the extent of variation in the visualrepresentation, could be performed using any conventional mappingprocess. For example, the values of surface scatter or haze could beassigned into values for extent of variation using any known technique,such as interpolation, or they could be assigned using a step 278 withreference to distributions of the scatter intensity values:

Referring to FIG. 95, there is shown a plot 279 of the distribution ofscatter intensity values on a wafer, by the percentage of a wafer's areaat which a scatter intensity value was measured, for the outputassociated with three collectors. The plot 279 can be developed for asingle wafer or for a set of wafers: preferably over a set of wafersrepresenting a process or related processes. The scatter intensityvalues for the output associated with three collectors are measured inphotons observed in parts per million (ppm). The plot 279 specificallydisplays the scatter distributions associated with a low surfacestructure spatial frequency range 283 (in the embedment described above,from the output associated with the front collector 330), for a mediumsurface structure spatial frequency range 282 (from the outputassociated with the center collector 320), and for a high surfacestructure spatial frequency range 281 (from the output associated with aback collector 340A, 340B, alone or in combination).

The plot 279 shows minimum and maximum scatter intensity values for thelow surface structure spatial frequency range 283, medium surfacestructure spatial frequency range 282, and high surface structurespatial frequency range 281, and it identifies the scatter valueassociated with the greatest percentage of locations on the wafer or setof wafers, respectively, the most frequent low surface structure spatialfrequency scatter intensity value 943, the most frequent medium surfacestructure spatial frequency scatter intensity value, and the mostfrequent high surface structure spatial frequency scatter intensityvalue 941. The most frequent surface structure spatial frequency scatterintensity values 941, 942, 943 are assigned the median value for theextent of variation in the visual representation, respectively medianlow surface structure spatial frequency variation value 303, medianmedium surface structure spatial frequency variation value 302, andmedian high surface structure spatial frequency variation value 301. Inthe embodiment in which color is used to represent surface structurespatial frequency range, the most frequent surface structure spatialfrequency scatter intensity values 941, 942, 943 are assigned the medianpixel color value for the display. In a display system which provides256 levels of a color, the most frequent surface structure spatialfrequency scatter intensity values 941, 942, 943 are assigned the medianpixel color value 256/2, which is 128. The surface structure spatialfrequency scatter intensity values above and below the most frequentsurface structure spatial frequency scatter intensity values 941, 942,943 but within the respective surface structure spatial frequency ranges281, 282, 283 are then assigned to the values for the extent ofvariation in the visual representation using any known technique, suchas interpolation.

Assigning of surface structure spatial frequency scatter values intovalues for extent of variation with reference to distributions of thesurface structure spatial frequency scatter intensity values could beperformed using the surface structure spatial frequency scatterintensity values distribution from the wafer under investigation, or itcould be performed using surface structure spatial frequency scatterintensity values from a plurality of wafers, for example, from aproduction ran of wafers having the same characteristics as the waferunder investigation. Using a plurality of wafers, nominal surfacestructure spatial frequency ranges for surface scatter may be identifiedand applied to the ranges of the extent of variation in the visualrepresentation.

As noted above, surface structure spatial frequency distributions may beanalyzing by forming graphical displays of the surface structure spatialfrequency distributions. Accordingly, the step 268, in which the surfacescatter is analyzed by its associated surface structure spatialfrequency range, further comprises a step 930 of forming graphicaldisplays to present scatter intensity associated with surface structurehaving a spatial frequency within a selected surface structure spatialfrequency range and the intensity's statistical characteristics at thedisplayed spatial frequency ranges, such as median or mean. In amulti-collector surface inspection system such as system 10, thegraphical displays would comprise graphical displays of the outputs ofselected collector. They could comprise charts or, preferably,histograms or bar charts in which the width of the bars represents theextent of the range of the surface structure spatial frequency responsefor the output of the collector.

The step 930 of forming graphical displays could further comprise aforming a uncomposite graphical display in which the output of theselected collector or collector and the output's statisticalcharacteristics is displayed at at least two surface structure spatialfrequency ranges. The composite graphical display could comprise acombined view histogram or bar chart, with each bar on the histogramrepresentative of a surface structure spatial frequency level. Histogrambars may be placed on the graph by increasing or decreasing frequencylevel, and overlaps in collector response ranges in surface structurespatial frequency levels could be shown by overlaps in bars.

An example of a combined view histogram is shown in FIG. 80, whichpresents a graphical display of the output data that formed thefrequency level haze maps of FIGS. 78A, 78B, and 78C. In FIG. 80, thesurface structure spatial frequency ranges defining high and mediumsurface structure spatial frequency levels overlap. The combined viewhistogram of FIG. 80 comprises a form of power spectral density (PSD)plot, in which power (represented by scatter intensity measured in ppm)is plotted in terms of surface structure spatial frequency.

As noted above, collectors are placed such that they receive lightscattered by surface structures having a spatial frequency within acertain surface structure spatial frequency range. In general, therelative proportionality of surface structure spatial frequencies foreach collector may be derived for wafer surfaces having definedcharacteristics. When nominal responses by a collector or set ofcollectors are established for surfaces having defined characteristics,any deviation from the nominal response of a collector response for atest surface having the defined characteristic indicates a possibleabnormality in the surface. For example, it could indicate a surfacedefect in or on a surface structure, due to a change in the productionrun in which the workpiece was produced.

Since changes in surface structure spatial frequency contributions froma workpiece surface may identify changes in workpiece production,deviations from nominal collector responses may be used to monitorworkpiece production. Baseline or norns comprising acceptable ranges ofscatter intensity measurement values for a collector in a definedposition in the space above a surface could be developed for arepresentative set of workpieces having specified characteristics.

For example, for a collector in a multiple collector surface inspectionsystem such as system 10 and in a defined position above the workpiecesurface, nominal responses may be developed from data obtained byoperating the collector to observe scatter from the surfaces of severalworkpieces, for example, workpieces produced using the same productionprocess. Minimum and maximum values could be determined in the ranges ofacceptable scatter intensity measurement values for each selectedcollector. In addition, baseline or norms comprising ranges ofacceptable scatter intensity measurement values could be developed for aset of collectors in defined positions in the space above a surface forworkpieces sharing specified characteristics.

The baseline or norms could be associated with an acceptable level ofsurface roughness on a workpiece of a specified characteristic. Thebaseline or norms could then be used to define norm value ranges for thescatter intensity measurement values in the output of the selectedcollector or collectors for workpieces sharing specifiedcharacteristics, such as wafers in a production run. In a subsequentproduction run, deviations from the norm value ranges in the output ofthe selected collector or collectors could then be used to indicateproblems in the production run.

Nominal ranges could be developed using the following method, shown inFIG. 100:

-   -   Step 931: Distinguishing characteristics (such as wafer type,        production type, polishing process, wafer annealing, epitaxial        processing, grain size)) are identified for the wafers to be        analyzed.    -   In a step 932, a set of surface structure spatial frequency        ranges is selected for the surface structures to be observed.    -   Step 933: The collectors in a multi-collector surface inspection        system such as system 10, which will provide output        representative of surface scatter sort-able by surface structure        spatial frequency range associated with the surface structures        to be observed, are selected to provide output associated with        the selected surface structure spatial frequency ranges.    -   Step 934: Output comprising scatter intensity values is obtained        for each of the selected collectors from a plurality of wafers,        for example, from a production run of wafers having the same        characteristics as the wafer under investigation. As described        above, output of selected collectors may be combined to create        output to be associated with a channel.    -   Step 935: Nominal scatter intensity values, for example in PPM        units, are developed for wafers sharing the distinguishing        characteristic for each selected collector (and thus each        selected surface structure spatial frequency range), and are        used to develop nominal scatter intensity ranges.

Once nominal scatter intensity ranges are developed for the set ofselected surface structure spatial frequency ranges for the set ofdistinguishing characteristics of wafers to be analyzed, multiplesurface structure spatial frequency haze analysis may be performed on aworkpiece. As shown in FIG. 101, one embodiment of a surface structurespatial frequency-based haze analysis method follows:

-   -   Step 936: A test wafer is selected for analysis. The wafer's        distinguishing characteristics are identified, and the nominal        scatter intensity ranges associated with wafer of the        distinguishing characteristics are identified.    -   Step 937: A surface inspection system such as system 10 is used        to obtain surface scatter patterns for the wafer from the        selected collectors. The system 10 scans the wafer using the        methods described above to identify surface scatter patterns.    -   Step 938: The surface scatter associated with the wafer is then        analyzed by its associated surface structure spatial frequency        range in the manner described above.

As noted above, the architecture of the multi-collector surfaceinspection system 10 supports providing a visual presentation of data inhaze maps from multiple collectors based on surface structure spatialfrequency content. One haze map could be used to show the entiredistribution of surface structure spatial frequency content (SFC) forthe system 10, with high SFC from the back collectors 340A, 340B, midSFC from the center collector 320, low SFC from the front collector, midto low SFC from the wing collectors 310A, 310B, and very low SFC fromthe light channel 650. Given a measurement system in a state of control,analysis of haze response over multiple scatter fields facilitates waferquality control that can cater to the substrate's end use and/orrequired channel sensitivities.

Analysis of haze response in which the surface structure spatialfrequency content associated with the haze is a variable encompassesqualitative and/or quantitative approaches. As an example of aqualitative approach, if a wafer produces a preponderance of hazeassociated with lower surface structure spatial frequency in one regionand a preponderance of haze associated with medium surface structurespatial frequency haze in another region, the lack of uniformity of hazeassociated with different spatial frequency ranges might indicate thatpolishing uniformity is not ideal.

Lack of uniformity of haze between different surface structure spatialfrequency ranges would be more easily observable in the embodiment of acomposite haze map in which scatter variation is shown by color. In theexample, one color would be present in one region and another colorwould be present in another region. If the haze readings were withinnominal (standard) ranges, the surface of the wafer would be uniformlycolored. For example, if the haze responses shown in a composite hazemap were within nominal (standard) ranges, the uniform red/green/bluecolors would blend together to present a wafer of a uniform gray color.

As another example, scratches, that could be caused by a number ofproblems, such as poor polishing, would be more apparent in compositehaze maps in which haze is separated by surface structure spatialfrequency than in haze maps showing haze associated with only onesurface structure spatial frequency range (i.e., a haze map showing hazeresponse for only one collector) or haze maps showing haze not separatedby surface structure spatial frequencies. Scratches appear in haze mapsas lines; the deeper the scratch, the stronger the line in the map.Certain scratches may be apparent in the haze associated with only onesurface structure spatial frequency range or in only a limited number ofsurface structure spatial frequency ranges, and they may not be apparentat all in the haze associated with another surface structure spatialfrequency range. Therefore, a scratch that is apparent in the hazeassociated with one surface structure spatial frequency range will notbe displayable in a haze map for haze associated with other surfacestructure spatial frequency ranges. In addition, in output of haze notseparated by surface structure spatial frequencies, signal associatedwith a scratch that is apparent in haze of a limited number of surfacestructure spatial frequency ranges is attenuated by the signals of hazeassociated with the other surface structure spatial frequencies in whichthe scratch is not apparent. The aggregation of signals in the displayof haze that is not separated by surface structure spatial frequencywill result in a signal in which the scratch is dim or not apparent atall. On the other hand, in a composite haze map in which haze isseparated by surface structure spatial frequency, the scratch could bepresented using a plurality of representations, for example in aplurality of colors, and the scratch will therefore will stand out inthe physical representative of the surface structure spatial frequencyranges of haze in which the scratch is apparent (for example, a lightgreen line will show up in a field of soft red and green haze).

In one embodiment, analysis of haze response in which the surfacestructure spatial frequency content of haze is a variable comprisesdisabling selected portions of the surface scatter intensity ranges inoutput associated with an individual contributing collectors. Disablingportions of a surface scatter intensity range essentially comprisessubdividing data associated with a surface structure spatial frequencyrange with which a collector is associated into scatter intensitysub-ranges so that haze may be analyzed in even smaller sets of data.After scatter intensity range sub-division, the data associated with thescatter intensity sub ranges may be used as the data associated with thesurface structure spatial frequency distributions in haze analysis.

With sub-divided scatter intensity frequency ranges, a haze map may beconstructed that shows how scatter magnitudes (in ppm units) aredistributed throughout the individual scatter intensity ranges inrelationship to the other surface structure spatial frequency responseranges. By disabling a certain range of scattered power over a selectedsurface structure frequency response collector, can generate a mapshowing the absence of the eliminated scatter intensity ranges in a map.While scatter from surface structure is more readily differentiated whenit is analyzed by selected surface structure spatial frequency ranges,signals associated with the scatter is still integrated within theselected surface structure spatial frequency range. Subdividing theselected surface structure spatial frequency range into sub-rangesassociated with the scatter intensity within the range provides smallerintegration and facilitates differentiation, even within a surfacestructure frequency range. With subdivided scatter intensity ranges, itis possible to remove a range of scatter intensities from a haze map toshow only some the presence of only some intensity values within asurface structure spatial frequency range.

An example of the utility of subdividing selected surface structurespatial frequency ranges is shown in FIGS. 96 and 97. Referring to FIG.96, a medium surface structure spatial frequency range haze map 939,comprising a haze map associated with the medium surface structurespatial frequency range, could show scatter of no discernible pattern.However, when the medium surface structure spatial frequency range issubdivided and a modified medium surface structure spatial frequencyrange haze map 941, such as in FIG. 97, is constructed, haze associatedwith all scatter intensity values in the medium surface structurespatial frequency range except the higher scatter intensity values couldbe displayed. It can be seen that map 941 shows a cluster of haze eventsin the center of the map. Such a haze pattern could indicate thatlocations in the center of the wafer are producing more haze than arelocations elsewhere on the wafer.

Haze produced by the center of a wafer could result from increasedamplitude of surface structures in the center of the wafer, which couldarise from over-polishing on the outside of the wafer and incompletepolishing in the center of the wafer, which, in turn, could arise from adeformation of the wafer while it is being polished. It is known thatbowing in the center of a wafer during polishing could arise from unevenpressures on the wafer platen. In response to seeing increased hazeevents in the center of a wafer map, a production manager could tunepressures on the platen in order to eliminate the deformation. Thussurface structure spatial frequency range-based haze analysis could beused to facilitate monitoring of wafer production.

Subdividing other surface structure spatial frequency ranges, creatingmodified surface structure spatial frequency range haze maps, andcombining the modified surface structure spatial frequency range hazemaps into modified composite haze maps could highlight with even greaterspecificity haze events that could be used to diagnose structuralconditions and processing problems.

Surface structure spatial frequency range-based haze analysis can beparticularly useful in conducting production problem troubleshooting. Inthe example described above, in which pressures on a platen caused waferdeformations that resulted in non-uniform polishing, confirmation of theexistence of a similar but aberrant scatter pattern in another frequencyrange indicated the presence of a global problem with the surfacestructure irrespective of the surface structure spatial frequency range.On the other hand, a composite haze map or haze maps of differentsurface structure spatial frequency ranges that show that all but one ofthe response collectors is observing uniform haze could indicate adifferent problem, such as polishing being uniform within some surfacestructure spatial frequency ranges but not all surface structure spatialfrequency ranges. A production manager, using the haze maps to identifyde-correlated data or data with low correlation, could then concentrateon other production issues, such as deficiency in the size of apolishing slurry or a chemical reaction.

If the production manager using surface structure spatialfrequency-based haze analysis identified a scatter pattern in the middleto high surface structure spatial frequency response ranges (in datafrom the center or back collectors) but not from the low surfacestructure spatial frequency range, (in data from the front collector, heor she could suspect certain issues such as problems with the slurry,which are more likely to introduce scatter associated with middle orhigh surface structure spatial frequencies. On the other hand, if theproduction manager identified a scatter pattern only in the low surfacestructure spatial frequency response range, he or she could eliminateslurry issues and focus on problems that are more correlated with lowsurface structure spatial frequency scatter, such as problems withholding the wafer (i.e. a defective gripper).

A multi-collector surface inspection system such as system 10 isparticularly advantageous in that it is capable of being used to analyzesurface structure scatter. FIG. 77 is a block diagram showing methods ofanalyzing surface structure scatter analysis according to the presentinvention, in which the optical collection and detection subsystem 7provides output associated with each collector detection module 200. Theoutput may then be used to perform angle-resolved scatter haze analysisor total integrated scatter haze analysis.

As described above, each collector detector module 200 is positionedabove a surface workpiece in order to respond to scatter associated witha particular spatial frequency range for a surface structure. Becausethe response range is constant within a given measurement configuration,e.g. incident beam angle, wavelength, collector dimensions, etc, theoutput from the module 200 may be used to perform angle-resolved scatterhaze analysis, with the back collector modules 340A, 340B observingscatter associated with the high surface structure spatial frequencyrange28 1, the center collector module 320 observing scatter associatedwith the medium surface structure spatial frequency range 282, and thefront collector modules 330 observing scatter associated with the lowsurface structure spatial frequency range 283. Since the scatter is thusobservable according to the surface structure spatial frequencyassociated with it, angle-resolved scatter haze analysis produces ameans of monitoring the various spatial frequency contributions tosurface roughness scatter. Collectors may be combined into haze fieldsto minimize the effects of incident beam orientation to the siliconsurface, e.g. combined back collectors and combined wing collectors.Visual representations such as composite haze maps may be developed toassist in analysis.

The output from the module 200 may also be used to perform totalintegrated scatter haze analysis, with the output form all of thecollectors summed, the reflected power measured and RMS roughness valuesproduced as described above. Visual representations such as single RMSmaps may then be developed to assist in haze analysis. Single RMS mapswould look very similar to haze maps, with map positions beingassociated with locations on the region under investigation, and witheach map position having a graphical element representing an extent ofthe single RMS roughness associated with the location on the waferassociated with the map position.

As noted above, typically, surface inspection tools that measure RMSroughness have a normal incident beam and obtain RMS roughnessmeasurements by obtaining measurements of the Total Integrated Scatter(TIS) from the wafer. When such tools comprise multi-collector tools,they obtain RMS roughness measurements from the haze observed across allof the collection optics, by summing the scatter output associated withall of their available collectors.

The system 10 differs from other surface inspection tools that measureRMS roughness in that it has an oblique incident beam and it collectsscatter from collectors disposed at selected angles. The obliqueincident beam and the angular positioning of its collectors introduce asurface structure spatial frequency component to the surface scatteroutput that provides improved haze analysis. However, system 10 couldalso be operated as a total integrated scatter tool, obtaining TISmeasurements by summing the scatter output associated with collectors300.

It should be noted, though, that aspects of the architecture of thesystem 10 cause RMS roughness measurements developed by the system 10 tonot match the RMS roughness measurements developed by typical RMSroughness surface inspection tools using normal incident beams. Thesurface structure spatial frequency response ranges of certain of thecollectors 300 to overlap, thus causing some “double counting” ofscatter when the output of the collectors is simply summed. Therefore,because of double-counting, the RMS roughness measurements developed bythe system 10 will not match the RMS roughness measurements developed bytypical RMS roughness tools. However, RMS roughness measurementsdeveloped by the system 10 will strongly correlate RMS roughnessmeasurements developed by typical RMS roughness tools.

While, as noted above, it is not necessary in angle-resolved hazeanalysis to include data from all collectors 300, in order for system 10to obtain strong correlation of roughness measurements with those oftypical RMS roughness tools, the output associated with all collectorsshould be summed.

For example, as noted above, data associated with the wing collectorsoperating in the P configuration may be excluded from angle-resolvedhaze analysis in order to reduce extent of overlap. Such a practice isacceptable in angle resolved haze analysis because most surface scatterwould be filtered from wing data due to polarization. However, in orderto obtain strong correlation of roughness measurements with typical RMSroughness tools, it is useful to include scatter associated with wingcollectors operating in P configuration.

The above invention has been described in terms of it use in theanalysis of unpatterned wafers. However, it is to be understood that theinvention is not limited to use in the analysis of wafers. The inventioncould be applied to the analysis of any suitable workpiece, such asglass and polished metallic surfaces and film wafers.

CONCLUSION

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details, representative devices and methods,and illustrative examples shown and described. Departures may be madefrom such without departing from the spirit or scope of the generalinventive concept as defined by the appended claims and theirequivalents.

1. An optical collection system for use in a surface inspection systemfor inspecting a surface of a workpiece, the surface inspection systemhaving an incident beam projected through a back quartersphere andtoward a desired location on the surface to impinge thereon to createreflected light, extending along a light channel axis in a frontquartersphere, and scattered light, the incident beam and the lightchannel axis forming an incident plane, the optical collection systemcomprising: a wing collector positioned to collect a first portion ofthe scattered light, the wing collector being disposed in the frontquartersphere, outside the incident plane, and at a null of abi-directional reflectance distribution function when the incident beamis P polarized, and at least one of the following collection elements: alight channel collector, positioned in the incident plane tosimultaneously receive the reflected light, or a front collectordisposed in the front quartersphere and positioned to simultaneouslycollect a second portion of the scattered light.
 2. An opticalcollection system as recited in claim 1, wherein the bidirectionalreflectance distribution function has a P-polarization component, andthe wing collector is disposed at a null of the P-polarization componentof the bidirectional reflectance distribution function when the incidentbeam is P polarized.
 3. An optical collection system as recited in claim1, wherein the wing collector elevation angle position is dependant onthe index of refraction of a workpiece.
 4. An optical collection systemfor use in a surface inspection system for inspecting a surface of aworkpiece, the surface inspection system having an incident beamprojected through a back quartersphere and toward a desired location onthe surface of the workpiece to impinge thereon to create reflectedlight, extending along a light channel axis in a front quartersphere,and scattered light, the incident beam and the light channel axisforming an incident plane, the optical collection system comprising:first wing collector and second wing collector positioned to collect afirst portion of scattered light and a second portion of scatteredlight, the first wing collector and second wing collector being disposedin the front quartersphere, outside the incident plane, and at a null ofa bi-directional reflectance distribution function when the incidentbeam is P polarized; and at least one of the following collectionelements: a light channel collector, positioned in the incident plane tosimultaneously receive the reflected light, or a front collectordisposed in the front quartersphere and positioned to simultaneouslycollect a third portion of the scattered light.
 5. An optical collectionsystem as recited in claim 4, wherein the signal comprises aP-polarization component, and at least one of the first wing collectorand the second wing collector is disposed at a minimum of P-polarizationcomponent of the signal to noise ratio when the incident beam is Ppolarized.
 6. An optical collection system as recited in claim 4,wherein the first wing collector and the second collector aresubstantially identical.
 7. An optical collection system as recited inclaim 4, wherein the first wing collector and the second wing collectorare located symmetrically with respect to the incident plane.
 8. Anoptical collection system as recited in claim 4, wherein the first wingcollector and the second wing collector are equidistant from the desiredlocation.
 9. An optical collection system as recited in claim 4, whereinthe first wing collector has an azimuth angle with respect to the lightchannel axis of about 5 to 90 degrees, and the second wing collector hasan azimuth angle with respect to the light channel axis of about −5 to−90 degrees.
 10. An optical collection system as recited in claim 4,wherein the first wing collector and the second wing collectors areequidistant from the surface of the workpiece.
 11. An optical collectionsystem as recited in claim 4, wherein the first wing collector and thesecond wing collector have elevation angles with respect to the surfaceof the workpiece of about 30 to 90 degrees.
 12. An optical collectionsystem as recited in claim 4, wherein the first wing collector and thesecond wing collector have elevation angles with respect to the surfaceof the workpiece of about 45 degrees.
 13. An optical collection systemas recited in claim 4, further comprising a detection subsystemoperatively coupled to the optical collection subsystem that detects thescattered light and generates signals in response, the detectionsubsystem having a wing detection component that is associated with eachwing collector; and polarizing beam-splitter disposed in an optical pathof the scattered light between the desired location and at least onewing detection component.
 14. An optical collection system as recited inclaim 13, wherein the detection subsystem further comprises a polarizingrelay assembly, wherein the polarizing relay assembly comprises avirtual mask.
 15. An optical collection system as recited in claim 14,wherein the virtual mask comprises a light absorbing glass maskoptically bonded to the polarizing beam-splitter or a black anodizedaluminum sheet metal mask positioned at a known optical imaginingposition in the at least one detection component.
 16. An opticalcollection system as recited in claim 15, further comprising a slit atthe Schimempflug angle corresponding to the angle of the wing collectorwith respect to a workpiece normal, wherein the slit is positionedbetween objectives lens and the polarizing beam-splitter and wherein thevirtual mask is located after the slit.
 17. An optical collection systemas recited in claim 4, wherein the optical collection system comprises amaximum of two wing collectors.
 18. An optical collection system asrecited in claim 4, wherein the one of the following collection elementscomprises the light channel collector.
 19. An optical collection systemas recited in claim 4, further comprising a central collector.
 20. Anoptical collection system as recited in claim 4, further comprising atleast one back collector.
 21. An optical collection system as recited inclaim 4, wherein each of the first and second wing collectors has acollection angle of up to about 40 degrees.
 22. An optical collectionsystem as recited in claim 4, wherein each of the first and second wingcollectors has a collection angle of about 26 degrees.
 23. An opticalcollection system as recited in claim 4, wherein the wing collectorelevation angle position is dependant on the index of refraction of aworkpiece.
 24. A surface inspection system for inspecting a surface of aworkpiece, the surface inspection system comprising: a scanning beamsource for directing an incident beam through a back quartersphere andtoward a desired location on the surface of the workpiece at an obliqueangle with respect to the surface to impinge thereon to create reflectedlight, extending along a light channel axis in a front quartersphere,and scattered light, the incident beam and the light channel axisforming an incident plane; a workpiece movement subsystem that moves theworkpiece relative to the incident beam; an optical collection subsystemcomprising a first wing collector and a second wing collector positionedto collect a first portion of scattered light and a second portion ofscattered light, the first wing collector and the second wing collectorbeing disposed in the front quartersphere, outside the incident plane,and at a null of a bi-directional reflectance distribution function whenthe incident beam is P polarized; and at least one of the followingcollection elements: a light channel collector, positioned in theincident plane to simultaneously receive the reflected light, or a frontcollector disposed in the front quartersphere and positioned tosimultaneously collect a third portion of the scattered light; adetection subsystem coupled to the optical collection subsystem thatdetects collector output and generates signals in response; and aprocessing subsystem operatively coupled to the detection subsystem forprocessing the signals.
 25. A surface inspection system as recited inclaim 24, wherein the bidirectional reflectance distribution functionhas a P-polarization component, and the wing collector is disposed at anull of the P-polarization component of the bidirectional reflectancedistribution function when the incident beam is P polarized.
 26. Asurface inspection system as recited in claim 24, wherein the first wingcollector and the second wing elevation angle positions are dependant onthe index of refraction of a workpiece.
 27. A surface inspection systemas recited in claim 24, wherein the detection subsystem furthercomprises a polarizing relay assembly, wherein the polarizing relayassembly comprises a virtual mask.
 28. An optical collection system asrecited in claim 27, wherein the virtual mask comprises a lightabsorbing glass mask optically bonded to the polarizing beam-splitter ora black anodized aluminum sheet metal mask positioned at a known opticalimagining position in the at least one detection component.
 29. Anoptical collection system as recited in claim 28, further comprising aslit at the Schimempflug angle corresponding to the angle of the wingcollector with respect to a workpiece normal, wherein the slit ispositioned between objectives lens and the polarizing beam-splitter andwherein the virtual mask is located after the slit.
 30. A method forinspecting a surface of a workpiece, the method comprising: scanning anincident beam on the surface of the workpiece to impinge thereon tocreate reflected light, extending along a light channel axis in a frontquartersphere, and scattered light, the incident beam and the lightchannel axis defining an incident plane; collecting a first portion ofthe scattered light at a wing collector disposed in the frontquartersphere, outside the incident plane, and at a null of abidirectional reflectance distribution function when the incident beamis P polarized; simultaneously collecting one of the following: a secondportion of scattered light at a front collector disposed in the frontquartersphere, or the reflected light at a light channel collectorpositioned in the incident plane to receive the reflected light;detecting collector output and generating signals in response; andprocessing the signals to obtain information about the surface.
 31. Amethod as recited in claim 30, wherein the bi-directional reflectancedistribution function has a P-polarization component, and the wingcollector is disposed at a null of the P-polarization component of thebi-directional reflectance distribution function when the incident beamis P polarized.
 32. A method as recited in claim 30, wherein the beamscanning comprises directing the incident beam through a backquartersphere and toward the desired location on the surface of theworkpiece at an oblique angle with respect to the surface.
 33. A methodas recited in claim 30, wherein the wing collector elevation angleposition is dependant on the index of refraction of a workpiece.